I need cheap Hp gains for my 2007 chevy cobalt.?

I would buy a header, they are basically they same length as an exhaust manifold, but they flow 10x better, and make about 10-15 hp improvement. A thermocooler won't hurt and advance auto sells one for like $30 they header will run you between 150 and 250 depending on where you buy it, just remember they're all just pipes, you can make a set yourself if you can weld.