Well, it's hard to say with the crazy prices these days. I would be okay with something like 45m if he left in 2013. 37m is too low with such a long contract (2016), I'd have wished for a 50m release clause to scare other teams off, though. But I guess it doesn't matter, anyway. If teams like Real, Chelsea, City or PSG were seriously interested and Götze would want to leave, it wouldn't make a difference if it's 40m or 50m.
Edit: I'd obviously wouldn't sell him for any price, though. Unless his little brother Felix is even better than him, then sell shit Götze for a high price