Cheap? A bigger cam. A 276* with HD valve springs would give you a bunch more HP, and a SRI would give you more top end, too. You'd probably gain like 15-20bhp there over the 260 cam and the stock ported mk4 manifold.
Ideally? A bigger cam (268/260 or a 276* 114*lc) with HD springs and a turbo kit from C2 or a BBM kit.. Or a Neuspeed kit. Lots of gains with FI on these little 8v's.
Wife has a Neuspeed S/C on her MK4 with a 268/260, 2.5" exhaust, and a few other mods and it's a nice smooth car to drive even with the automatic. It's probably around 145whp or so. Not fast, but not slow. Beats my friend's stock 1.8T 5 speed.