how much will it cost to convert the air suspension to a coil, spring non air system?

Check out this company: http://www.strutmasters.com or call them at: 1-866-597-2733 They have what you are looking for. Zee

What you do is go to your parts store and look up standard rear coil springs for a 1995 Mercury Grand Marquis. When I did our Lincoln the set cost $285. You can just tie the air lines from the compressor to the frame above the axle to get them out the way, rather than cutting them off. Remove the fuses to the air suspension computer in the trunk and the air compressor on the engine. Just take out the airbags and install the regular springs from the Mercury. The car rides just as good and you'll be done with that tail-draggin' stuff.

get springs at around 65 bucks, easy to do. takes about one half an afternoon if that long