Christ, I paid £35 for 4200 on Monday. Seems quite expensive but you can get enough for your money. Considering a new game costs roughly the same depending on how new it is.
I bought another 4200 when TLAD came out, the rest of the points then I bought all of the Forza 2 DLC, PGR4 content and some other little things that I don't remember.
This time I bought Trials HD, a few car packs for TDU and i've still got just under 2k points left over.
Edit: 248 attempts, 23 minutes later and I finally got to the end of that King of the Hill level.