Forza would not be an annual series if MS were spending $100m per game lol. It's just basic common sense that they don't spend a huge budget really.
You're evaluating it as a "normal" game bound by normal performance considerations. I suspect this is a mistake. Forza, like say Killzone, exists as a "must" have exclusive franchise where the console provider feels they're weaker than the competition.
This means it's much more likely to get unduly high funding/marketing vs actual sales (and to be blunt I'm not just talking FH2 - look at advertising presence for Forza in general and the high level of bundling as well as the undeniable quality and high investment in each title and it's obvious that MS are investing heavily in the title as a key pillar franchise to help them grow internationally.
I doubt in total Forza has returned much profit at all for MS - not saying it's not profitable but there's zero reason to see it as some cheap to produce franchise that turns a high profit..
The good news is as such the franchise isn't bound to normal succeed/fail criteria - so long as MS holds an hope of doing well in Europe they will likely invest in the franchise particularly if the quality remains high (because it is a very good franchise).
The bad news though is that, on current trends, this is going to be a brutal gen for the franchise sales wise. Xbox is showing severe retraction in Europe (and UK for those that see it separately) as a brand and this is where the franchise should have highest appeal as a genre so it's going to have an uphill struggle this gen for sales/marketshare.
For example I wouldn't be surprised if Driveclub outperforms it merely due to platform of sale vs anything to do with DriveClub quality (good or bad though they game may be).
I'm not just trying to bash here though - I've always wondered whether Killzone has ever turned any significant profit either. It clearly costs a lot from what I've read and is advertised and bundled heavily just like Forza as Sony tried to grow it as an FPS franchise to compete with MS dominance (in general previous two gens) in that genre. Forza is MS attempt to compete with Gran Turismo dominance and give their customers a viable alternative if they forego Playstation and by extension access to GT.
I really think you're viewing the franchise wrong, including likely FH2 costs and profitability.