Figure for $25K you could have a built VW transmission, turbo VW engine, fab up some a-arms, Wilwood brake parts (front, VW Wilwood brake set rear), Erco VW wheels (7lbs/each), and a chromoly Formula Vee style chassis. Wrap it in fiberglass. 400 hp out of a built, turbo'd VW engine isn't that difficult anymore. But reliability would be lacking.
Or, pick up a Kennedy Subaru/VW bellhousing adapter and clutch set up and throw in an STI mill, deal with water cooling, and you have a reliable track car.
I've put absolutely no thought into any of this