Definitely not a Cinelli. Cinelli never did seatstay caps like that, and wouldn't have left the dropout/stay connection unfinished like that either. If I had to take a fairly WAG, I's say either a small custom builder or a garage builder, maybe even someone who wasn't capable/confident in making a fork for it. Especially if the fork is original, I'd place the frame mid to late 90s.
As far as worth, I don't have a really good idea. The fixed gear craze has pushed the prices up for bikes like these; you may be able to get $500 dollars for it -- but you might only be able to make $300 or $350.