A good mate of mine competes in the msa tarmac rally series, he does pretty well and has won the series a few times for his class, even won events outright and that is up against genuine ex wrc cars and escorts running millington engines which cost 4 times his car, just for an engine. He has used various cars, 205, davrian, gtm and is now building a mr2 mk3. I am sure he wont mind me saying he is not a wealthy man and competes on a low budget doing most of the spannering himself with the help of mates and because he is successful has managed to pick up some sponsors, but they mainly donate cheap parts more than actual £.
The thing that strikes me, away from the cost, is the sheer amount of time required, forgetting the time at the actual event its the hours and hours spent working on the car, fixing, improving, maintaining etc etc, he is forever in his garage or going to collect parts and also space required, car, trailer, spare wheels and tyres. A massive all round under taking.
Speak to Andy James ref hillclimb, as per Keyser, its probably the easiest and cheapest to get into.
Brillo also makes a very good point re entry costs and actual time on track, i was chatting with my mate mentioned above about booking a Goodwood track day, i mentioned its not cheap at £350 in August and he said he did a rally there, £250 entry fee and did about 45 miles.