MK3 can be had for anywhere between $2,000 to $15,000 ($3,500 being the normal average), $15,000 being the car in the most pristine condition with so many mods done all over the place, including interiors, that I'd be wondering why the owner is getting rid of it in the first place. I know for a fact that one well known mk3 guy was selling his for above $20,000, but it had so much work done....
To be blunt, the mk3's are a headache. How do I know? I've owned one for years. That's what makes them fun as long as you KNOW what you're doing with them, since you'll be doing a lot of the work on your own. You'll definitely need to join an online Supra community for support.
As for an MK4, they're expensive as hell but kinda easy to find parts for them.
Supra: To surpass or go beyond.