Sticker price for all the cars is 205 million credits. However, that doesn’t take into account prizes, wheelspins etc, so you’ll need a lot less than that.
At present it’s impossible to get all the cars as the 2017 Ford F-150 Raptor Forza Edition hasn’t yet been unlocked. If you’ve played other Forza games, got a good Tier level and the Ultimate edition, credits aren’t really the main problem. What can be frustrating are FE cars locked behind challenges which a player has missed first time round and parts of the game such as the Message Center not working.
And when it is eventually released its not likely to cost anything either
shouldn’t be too fussed about FE cars anyway…can always use the non-FE version just without the credit bonus, no other difference