I doubt the Forza 3 development was more than £10mil considering the short development time and considering that, you would expect GT to be much better considering their timescale and budget being directly under Sonys feet developing a signiture title.
GTA is a HUGE franchise with a huge community with a lot of well grounded roots, and IV has been a huge leap and a long development. The engine has a lot of potential if they choose to use it some more. GTA has to answer for my part in online communities, I've been a member of GTAForums for 7 years and was part of other forums based around GTA1 a long time prior. Scary stuff.