Agree with a lot of what Andy said - unsurprising as we race with CSCC - did a few years with a 944 Turbo in Future Classics, and now run an Alfa 75 V6 in the same series....outclassed by the Porsches, but still enjoying it!.
These races are good if you can get to share - otherwise at 40 minutes they might be a bit much for a first race, and of course entry fees are higher - typically £385 for a 40 minute race. Good bunch of competitors, though, and driving standards are well enforced.
You will need to spend around £1k on basic kit to get started, as well as doing your ARDS course, after that its rather up to you. There are certainly cars around (buying a ready-prepared one is much more cost-effective!) for well under £5K, and if you avoid accidents and can resist the temptation to buy new tyres too often (we've had a season out of a set, but we aren't exactly cutting edge) you can keep costs down. Realistically though, if you compete at a range of circuits rather than just your local one, by the time you've added in travel, accommodation, fuel and a little socialising to the entry fees and routine maintenance, you are probably looking at about £1K per meeting, maybe a bit less. Can be much more of course if you outsource work on the car and are unlucky with the mechanicals.......
Trouble is it gets into your blood, like all motorsport, which is why after starting rallying aged 23 I'm still competing over 40 years later.......