There are still places left on all the StreetSafari rally's this year at time of writing.
I won't be doing one this year due to the imminent arrival of the latest addition to our family and other things, but I'll be doing one in 2008 providing there're no major disaster between now and then.
On S2n me and my two mates chomped through around £100 a day, and that's not including getting the car ready, but then we weren't really watching the pennies. It can be done really cheap if you're prudent, you might even want to consider camping to keep costs down.
I'd recommend three as a team in a large car is comfortable. (There were two guys did S2N in a Mini last year, I'm amazed they didn't go stir crazy!!)
I quite fancy the BA/BE (New York to New Orleans) rally, but that's might be a bit out of the question.
Shame I didn't still have the BatMobile, It could be the club rally car!!