Wow, 5 pages of discussion while I was away. No spoiler as of now, might come out later tonight or tomorrow. For Hayate's salary or allowance, its based on the main factor, his debt. We aren't sure whether Nagi covers the living expenses or the education expenses. Nagi (or was it Maria) said he need to work for 40 years in order to pay for his approximately 150,000,000 , so he approximately receives 3,750,000 or more and not counting on the other factor. If he had to support himself, that would mean he has to extend his debt and receive even less. If he doesn't have the debt, he actually earns more than any other ordinary jobs. With that said, it doesn't really matter if Hayate has any money since Nagi will buy whatever Hayate wants if he asks her.
Here's some info from Hata's diary on chapter 245, I'll list the potential interesting parts.
Volume 21's omake has Klaus and Yukariko's backstory.