We are getting a new house built and installing blowers in the 2 gas fireplaces is 250$ each.
Do I need a blower? Why would I want one?
IMO, I would want the blower. The convective flow of hot air out of my current gas fireplace is pretty slow, so if I wanted to heat the room or house with it, it would be a pretty slow and inefficient process. Also, without a blower, the fireplace front/mantle gets DARN hot! Hot enough to discolor paint. As far as blowers go, you want the 'squirrel cage' type, not the 'axial' fan type, the squirrel cage blowers develop more pressure and move air more effectively, from personal experience with a lousy axial fan!
Some gas FP models come with a blower as standard equipment. You might look at other models & compare features & prices.
Also, if you want the FP to operate in a power outage, make sure it says it's capable of starting the unit & running w/o electricity (blower won't run, but you'll get radiant heat).
Most buyers of gas fireplaces prefer a blower. They add significantly to the heat output with most gas fireplaces. There are some exceptions to this.... such as in 'Valor' brand Radiant Gas fireplaces.
What I'd also be asking myself is what make and model are the fireplaces? Most spec home builders put in the cheapest, stripped down 'Plain Jane' fireplaces that they can get their hands on, and then vastly over charge for any accessories or upgrades.
I'd recommend that you educate yourself a little bit by visiting a few local Fireplace Specialty stores and asking lots of questions. Look and compare features. They are vast. I can guarantee you that your builder doesn't really have much of a clue about different fireplaces. That's not his job. His job is to build & sell you a house and then go do it all over again! Many of your better quality gas fireplaces come with a blower already installed from the factory.
Here is a link that might be useful: valor fireplaces