I haven't looked at the wiki- but that "10 girls" seems kind of light. I'll admit I was only able to earn 3 girls from the darklord, and none from the ninja or troll- but thats 9 girls right there, plus the "halloween" specials from the dark lord (that I never found....), and the 5 or 6 you get for advancing the story...
It really shouldn't be a problem getting money to upgrade the girls. although lack of items to do it with can be(with 12 hour market resets, it can take 2 or 3 days sometimes to find the items to upgrade a girl to max).
you are going to just need to log on more often (upgrading the girls doesn't increase the total money they give you, the increase in amount is proportional to the increase in time between payments)
By the time the princess asks you to spend 250k, you'll hardly miss it