The only way to answer your question is to get several competitive quotes from reputable dealers. Make sure that they are quoting the same work, including any ductwork mods, replacing refrigerant lines, electrical, etc.

Remember that SEER is a laboratory-derived number. Many things in the real world, like air flow thru the ducts, refrigerant charge, and varying outdoor temps and humidity effect the SEER rating (and thus energy consumption).

An 18 SEER unit would be a 2-stage compressor. These systems are good at removing humidity when they run on low speed. Running on low speed is also where most of the calculated energy savings are derived. A variable speed matching air handler must be paired with a 2-stage condenser for proper efficiency.

A 12 SEER unit is not that old. Is it in good shape or are your trying to replace it solely for energy savings?

I'll guess $9k minimum for your new system.

The old 12 SEER unit just quit cooling and even when it was it didnt do a great job. I'm sure that replacing it with a more efficient model wont necessarily mean that it will cool better but the old one ran constantly and did a pretty mediocre job.
As I have to get it fixed anyways I thought I'd drop the money in a much more efficient system, but if $9k is anywhere close to correct I'll stick with the old unit.
I did get a quote for a 15 SEER at $5400, so you suppose that the difference between a 15 and an 18 or 19 SEER would be $3600+ dollars?

Id guess more like 2$k more for a seer 18. But 3 tons is supposed to be 3 tons and it shouldn't cool any better. Perhaps a manual j is in order to see if you don't need more tonnage.

less dehumidification with 18 seer and above in our climate where dehumidification is such an issue.
15-16 seer units are variable speed, no need to go higher.
I'd invest that added cost in variable speed & sealing ductwork. just my .o2

best of luck

Thanks for the input.
I'm trying to have the existing repaired for now. I wouldnt have believed how hard getting someone to my house would be. I scheduled two A/C companies/guys in the area, mostly found through Yahoo and other business ratings sites. Both guys are well rated but neither has shown yet.
One of the guys called me today and said he just missed me, said he had a complete install so was tied up for the day, could he help me now he asked. No thanks.
I called the other guy yesterday to say I was home and if he could make it early I'd be home. he asked "whos this again", he didnt have me scheduled. I scheduled with him the day before. Are you kidding me!

Does anybody know of a reliable A/C guy in the Garland TX area that wont rip me off?

installs are notorious for running over.
can't leave until you get it right, I'm sure
you would get behind this if it were your install!
the 'who is this' guy..not so much! it only takes
a minute to write down a name...

best of luck.