I've only ever bought packs/cars/tracks when they are on sale, IIRC I got the DTM 13 Pack which gave a load of tracks first, then the Touring Car Classics, then the Grp.5 if you wait for Store Sales/Specials you can put together a very decent amount of content really quite inexpensively. I spent around £18 and easily got a 'Full Game' worth of content.
The other trick is to buy a vRP voucher which works out more cost effective than buying cars or tracks in pounds sterling. You can buy the vouchers from here.