Good points.
Well, Rangers are in massive debt, so thats where most of there money goes.
Celtic used to be in debt but have came out of it, but they are reluctant to spend unnecessary money. I say unnecessary because they're not going to win the Champions League even if they spend a lot of money, and they could probably win the UEFA Cup if they play well and use the right tactics, although even winning the UEFA cup would be a great acheivement.
But domestically, they don't need to spend that much. Rangers and Celtic only have each other to beat. So they don't need to spend that much money to win the League. So why spend money you don't need too?
Of course, being Scottish, I would love to see someone else challenging for the title (preferably Hibs

) but sadly, its not going to happen because of the massive gap between the two groups (Old Firm and non-Old Firm).