Why limit your budget? If you have the cash flow available (or credit limit for that matter), why put a ceiling on your spend as long as you're still turning a profit?
As for spending, I've had some days with $800+ spends in the past few months. I've found some more effective ways that get higher ROIs (with much less effort in optimizing and maintaining campaigns), so I've let that drop a bit in the past month or so.
My budget is probably around $250 something a day between the big 3, but it's never even close to reaching that. Probably more like $10-15 a day max. I used to be able to spend around $75 a day when I was pushing ringtones hard tho.