Did you sign all the paperwork yet? You could try playing a little hardball and ask for a better price...Do you know what the actual upcharge would be? Maybe it's not as bad as you think. If you can find out maybe the guys here could advise if it's worth it if they knew the $$. I was told by an hvac service guy that the builder buys all of the same units for a development so it ends up that some are properly sized and the small homes get oversized and the big ones get undersized. Actually, they may buy all the biggest size. His point was they buy one model to get the cheapest price. Could be true, and could be what's hurting you, because yours wouldn't get a quantity discount. Our builder did offer a high eff furnace.In 1996 it cost us $800 to get a 91% furnace instead of the 78-80% that was standard. We replaced it at 17 years old. I think it was worth it...$50/year. I'm sure it saved us more than that! Good luck in your new home.

Given that this is new construction (so assuming energy efficient) and that you live in Texas (hot, humid) the equipment proposal sounds over-sized. They are proposing 7 tons when roughly 5 tons would appear more appropriate. Don't do anything until they justify the sizing with a detailed load calculation. Get the manufacturer names and model numbers fo the equipment they are proposing. Propane is generally the most expensive form of heating. Have you considred a heat pump?

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.

A fixed speed blower will always run at the same speed regardless of the static pressure. But a variable speed blower will increase the RPMs in order to maintain a constant air flow. If you have a duct work issues or a dirty filter, the amount of energy used by the variable speed blower could be more than the fixed speed. This will subtract from your energy savings. I personally would get the variable speed furnance for the extra $300.