I started with around 40 or 50m and was able to make a decent profit. I usually flipped archer rings, berserker rings, staffs of light, or barrows items.
There is no amount requirement to flip, you can start flipping with 1k ^^.
Eventhough I'd personally say 10m is enough to start, but once you've flipped with 500m, you struggle to flip with 200m
It's all about how confortable you feel.
Was able to flip 5m into 22m. So really depends on what you want to flip, the item and the market. 10m should be average/start for most. This was about 6 months ago so won't bother posting what I was flipping.