It depends on how many shocks are bad. It is usually about $700 per shock ( ouch, I know that is really a lot to deal with) Sometimes you can find decent used air shocks at a wrecking yard, but they still can cost $300 each, because the wreckers know what the new ones cost.
I have a 97 Town Car and it kept lowering shortly after I turned off the engine so I knew the air suspension was leaking especially since the car has 150K miles.
Anyway I checked the ads on our local craigs list here in Dallas and got everything replaced: parts and labor for 590.00... Way less than the dealership.
If you have air suspension replaced at a Ford/Lincoln/Mercury Dealer yes the cost is outrageous.
Again, I had mine replaced at my home for 590.00 including parts and labor and the car drives great.
Some people do not recommend replacing with springs since the car was designed with the air suspension to give it that "special" ride that Town Cars are famous for having. It is all a matter of personal preference.
The problem is that after a while, parts need to be replaced, air shocks leak eventually. But it does not require replacement very often so the cost is not bad.
Depending on where you go, cost is going to be more expensive. Dealer pricing is terribly expensive!!! Try Strutmasters. They helped me with my suspension problems. Maybe they have a fix for your problem. Great customer service!!! :) Good Luck