I would love it if when you didn't have enough cookies to buy 10 or 100 of a certain building, it just wouldn't let you buy any at all instead of buying as many as you can afford.
This has bit me in the butt so many times when I get trigger-happy on the mouse and end up spending way more cookies than I had intended to. Or when I'm trying to keep my number of buildings as a multiple of 10 to satisfy Rigidel (I know they don't have to be a multiple of 10, but for me it's just easier that way).
Not to mention the current process is simply a bad design, IMO. If you can't afford 10 buildings, the option is grayed out. That should mean that it doesn't work at all. Not that if you click on it, you buy 7.