I have a two-zone system and I think it was about $15,000 to replace last year (Carrier Infinity series, SEER 21 I think), but that's on the "gimme the frosting and everything!" end of the spectrum. I will say I've seen a noticeable reduction in my electricity bills, post-install.
Freon is highly controlled, so it's very expensive. Part of the reason I had my system replace (other than be-ing 15 years old) was that I could have recharged it with Freon, and a week later it might all leak out... and I live in Atlanta, where unreliable air-conditioning is not an option, and I didn't want a $500/week Freon habit.
As far as DYI goes, any R-22 available on the market is recycled; you also need en EPA certification to (legally) purchase it (likewise other refrigerants, R-134a, etc.)... I would approach offers to sell R-22 online with skepticism (FWIW).