You seem to be nice and honest guy... So my solution for you is, go and troll the biggest tubes, niche tubes and other similar sites which are popular ( like cutscenes.net or spankingtube.com) and repost to your tube anything you want as LONG AS !!!! the clips are short ( lets say up to 7 minutes) and stay away from ripped off longer clips or God forbid full videos.
Also credit ( burn in) the original website ( if you know it) or do some home work to find it and voila...
1) the copyright owners will leave you alone ( I certainly would)
2) you will have the niche or general content you want
3) you get traffic
And then when all is done and you are as big as xHamster you can pay/per click to the copyright owners and become a hero...
If you still insist on buying clips it can go from a dollar to 100x more depending how long, how exlusive, how good, how rated...etc...etc... Almost impossible to determine..to content owners it is about traffic... So if you have traffic then:
A) most of us would it give it to you for free
If not??
B) i would sell 10 min clip for about $ 100 with a minimum of 50 or about 5K ...this will seem very expensive but I am sure you can find folks here to give you the same for a dollar each...