You'll get the standard "there are a lot of variables" posts here, but here is what I have observed in the last 6 years or so of looking and watching them sell:
A good, clean 74-76 Cadillac ambulance that drives well and has little to no rust or body issues will start in the $5000-$6000 range. Probably have faded chrome on the lights and sirens (if visible), a good driver. Start looking at cars approaching show quality, with no rips in the interiors and excellent paint, and you're looking at $10K plus. If you're looking to get one restored to factory, probably double that, plus some. You're better off finding one somebody had recently restored.
If you specifically want a 75, rarity will come into play, since ambulance-only cars were on their way out of prduction starting in 74. 1976 is widely regarded as the last year of full-sized, ambulance only cars, with a scant few produced in 77-78. By 79 you could only get a full-sized combination car or hearse. After the early 80's full-sized car based ambulances and combinations were gone.
70-73 cars turn up much more frequently, and you can sometimes find a good deal on a restorable car for $2500-$3500, but it will need a lot of work, and parts aren't cheap.
Alot depends on being in the right place at the right time, too. I have seen more than a few decent mid-70's M-M cars get snatched up in the $3500 range, only to be resold for double with not much more than a wash, wax and maybe a tune-up. They certainly aren't getting more plentiful. Keep scouring sites like this and your local Craigslist as many times a day as you can. When they come up, they don't last long. There are also people that hunt these things down that can put you in touch with a seller if you're serious, for a small finders fee.