Maybe it goes to quality, but of the actual models I've priced so far, the 'real wood' burners run 30% to 50% more.

You may also find a big difference in venting costs. Gas can go right out the back wall, usually much cheaper than running a wood approved chimney out through the roof.

A gas fireplace is a sealed unit that only burns gas. If the BTU's are high enough they will heat your home.

What you are referring to as a "wood-burning insert" from your builder, is actually a pre-fab metal fireplace. A wood burning insert is a stove made to fit inside an existing woodburning fireplace.
Don't ever go for the builder's box pre-fab fireplace. They are huge pieces of junk, are usually installed improperly and should be outlawed.

Here is a post on this subject from 3/2:

RE: Direct Vent Fire place recommedations (Follow-Up #1)
posted by: mountainstoveguy on 03.02.2007 at 04:30 pm in Fireplaces Forum

all manufactures including lennox, make all grades of direct vent fireplaces
builder grade fireplaces all have this in common
tempered glass
not so attractive logs
not many options as far as fronts, refractory brick package etc.
mid grade fireplaces will have
tempered glass
good looking logs
some front and brick options
high end fireplaces have
ceramic glass
extensive ember beds
great looking logs
lots of front and brick options
price points
builder mid >$1000 high end >$2000 Mid grade and high end will usually have a decent efficiency rating, ceramic glass ones put out the most heat because ceramic glass transferrs infrared heat very well. Some good manufactures are:
fireplace extrodinare
heatilator caliber series
jotul acadia
lexington forge
heat n glo
mendota
lennox
majestic
im sure there are others, but all these manufactures offer every thing from builder grade to high end efficient models.
HTH

Here is another previous post on the subject:

posted by: renron on 12.19.2006 at 07:28 pm in Fireplaces Forum

First, DON'T install a ventless fireplace!!!
Even though the MFG.s will tell you the exhaust fumes are OK, do you really want to breathe them? Many people end up with Major Headaches and chronic breathing issues. Does it sound like a good idea to breathe burnt exhaust gasses?
B type flue(vent) pipes are dual thickness pipes. A pipe inside another pipe, suitable for use at 1" or more to flammable materials. Outer casing pipe is sealed and will not conduct air.
D (Direct Vent) type flue(vent) pipes are also dual layers but they are capable of exhausting burnt gas through the inner pipe and importing exterior(outside) combustion air for burning. Think of a straw inside a larger straw. Inside straw blowing, outside straw sucking outside air for combustion. A fan is usually used to help exhaust the burnt gasses.

Details below:
B-Vent (Natural Vent) gas fireplaces are designed primarily for decorative use. Generally they produce a larger and more realistic yellow flame. B-vent fireplaces are sometimes available without a glass window, so many homeowners are attracted to this design because of the open, realistic flame effect; some models do have a glass window so the existence of a window on a fireplace does not define its product classification. Natural vent gas fireplaces use room air for combustion and vent fumes through a vent or chimney that must terminate vertically above the roof line. The key to determining whether a B-vent fireplace will meet your heating needs is to check the manufacturer's listed efficiency rating (gas consumed x efficiency = heat output). While efficient, heater-rated B-vent fireplaces do exist, many lower quality "builder grade" fireplaces have no efficiency rating and, therefore, will not supply substantial heat. In fact, this category of fireplace will often use the greatest amount of gas at the lowest range of efficiencies. If you live in a warmer climate where you want the aesthetics of a fire without much heat, this may be the appliance for you. Likewise, bedrooms and smaller rooms may be a good candidate for a b-vent fireplace, but please check building codes to see if this class of fireplace is approved in your locality or for installation in the room where you'd like to use it. Local/national codes in some areas prohibit the use of b-vent rated appliances, especially in colder climates. Be aware that very "air tight" houses can cause performance problems with b-vent rated appliances, so please consult an experienced fireplace installer who can advise you on the proper installation and use you are considering.

Direct Vent Fireplaces offer the most features with respect to gas consumption, efficiency ratings and venting options. A direct vent fireplace will always have a glass window because these fireplaces are sealed systems using a double walled venting system. Combustion air enters the appliance via one section of pipe while fumes and moisture are vented through the other pipe. Most often, a double wall "pipe within a pipe" system is used. Subject to each model's requirements for distance and offsets, direct vent fireplace venting may be terminated either horizontally or vertically. Efficiency rating will average 65% to 84%, similar to ratings on gas furnaces. Keep in mind that these appliances must "waste" some of the heat produced to create a draft of rising warm air to evacuate the fumes produced without aid of a forced air exhaust system. Think of direct vent fireplaces as a decorative furnace: the beauty of a realistic flame with high efficiency. This appliance is great for primary or supplental heating and for emergency backup heating as most work without aid of electricity. Because this is a sealed system that uses outside air for combustion, direct vent fireplaces are usually the favored choice among those in the fireplace industry because of their greater efficiency and exceptionally reliable performance.
Hope this info helps,
I am a General Contractor.
Renron

When we built our house 8 years ago the model package we chose came with a woodburning fireplace. I took the advice of a friend and paid the 'upgrade' and had it installed with gas. Only after much consideration, discussion with others and finally visiting the fireplace contractor who installed the unit did I realise that I DO in fact have a woodburning fireplace with a gas-log setup installed in it. This was what I was hoping for. After about 30 minutes of work to remove the gas guts and install an ignitor kit, I now have the wood fireplace that I wanted for so long.
I guess my point is never say you'll never use a wood fireplace. Its not as convienent as gas but the cost of gas keeps rising.
Just my .02 cents worth

Dave