Depends on how you define a "flipper."
Wholesale deals (which isn't quite what you're describing): $5,000-$7,500, though it could be higher.
Deals in which the investor actually purchase the property, does some work on it, and then resells it: In the range of $20,000 or so. Anything less, and it's too risky--too much exposure, too much work, and not enough margin if things go wrong.
Hope that helps.