I think it really depends on how good of condition that $800 car is in. If it needs a new engine, then that'll run you some coin, and the sky is the limit on what you want do to with it. You can spend small fortunes on everything. Body work, paint, etc. I know guys that have blown $5000 on paint alone. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/notify.gif" border="0" valign="absmiddle" alt=':notify:'>
But for autocross, just need a good suspension setup (coilovers, swaybars, etc) and a grippy set of tires. This alone can blow away a good chunk of your budget too.
I'd just make sure the car's frame isn't cracked or bent, and slowly go from there. See what needs to be replaced right away, and budget from there. Take it to a mechanic (if you're not one) and see what it truly needs. Hard for us to say when we can't see it or feel it.
Good luck with your new project though. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'>