Time Attack and D1GB/Eurodrift is as small a budget and as least restrictive as you gonna get, and even that is expensive to be competitive, just cheap to get in to.
Tho TBH competition in the FWD category at TA is low at mo, and D1GB/EuroDrift is 80% skill and 20% car, so competitive is possible without big bucks.

Stuff like Stock Hatch etc etc is the cheapest aside from that, but to keep it cheap and competitive the rules are restrictive in what you can do to the cars, which may or may not take the fun out of things, depends what kind of person you are.
Hot Hatch aint cheap if you want to be competitive, so your better off staying in Stock Hatch unless your mildly minted.