Depends on what level you want to do it mate...
I dont know alot about saloon car championship... but if you want to compete on stage rally such as Robin Hood Stages in the HRCR then ur looking at £500 entry fees, a couple of sets of tyres per event at about £80 a tyre, plus travel, hotels for the weekend etc
The real cost comes to the damage to the car and the repair bills!!
Although, as I said, it depends on what level you want to do it and how competitive you want to be.
Road rallying is a far cheaper option.. with the emphisis more on navigation but with some quality off road sections.. entry is £60-£100 average.. and most go on through the night.... Alot less cost is car preperation as well.
It all depends on how far you want to take it.. both are fooking expensive I imagine if you want to be competitive.