I just picked up an E36 M3 for that very purpose. I don't think there is a better car for the budget minded enthusiast. Great feeling and handling car (it was car and driver's best at any price back when it came out, so even by today's standards it's not too shabby
) + parts and consumables are cheap and plentiful, 17" tires etc. And I can afford writing it off
My E92 is paid off but I'd cry if I totaled it on the track.
E46 M3 is another fantastic option if you can afford it. They are a little more expensive to run and maintain (larger/wider tires, vanos, rear subframe to name a few) but popular for a reason.
Sidenote: whoever mentioned an LS3- I can't think of a worse engine for a dedicated track car. Need serious cooling and oiling mods to be reliable.