Why is leeching at x location cheaper than location y? I mean, who cares if one place gives more in terms of loot. Time is time, a leech seller is doing the same job regardless of the location. Leechers, what are your opinions?
You also have to consider what drops the mobs have and the map layout. No one really wants to sell at teddies because there's no drops that are worth any value above their NPC price. And people also don't really want to sell at ulu 1 as opposed to ulu 2 because ulu 1 is a very vertical map that requires concentrated navigation while ulu 2 is just a simple back and forth ult spam.
You have to look at it from the point of view of the person selling leech. Say we're in your scenario where all maps have the same price per hour of leech of 50m. I'm a bishop that can 1-hit any mob in the game and you ask me to leech you for 1 hour at Jr. Wraiths for 50m. As a leecher, I know I will receive 50m from you, plus maybe 10-15m from NPCing mob drops and I know Jr. Wraiths don't have a chance to drop any godly end-game items. I will make 60-65m in that hour. Or, I can sell leech at Skeles for 50m/hour in your scenario and get 25-30m of NPC drops in that hour plus have a chance at godly King Cents, Spartas, warrior tops, and bowman overalls. That nets 75-80m guaranteed with potential of 150m+ for an hour. Why would a bishop ever elect to sell the wraiths leech when he could just as easily go over to skele and make considerably more money in the same amount of time? In order to offset this loss of npc drop money and loss of potential items worth selling in the fm, the leechee has to be willing to pay more for the wraiths leech or he will have no one willing to sell him leech because there are better options available. Not to mention, the bishop would rather leech in the high leveled map for better experience gain as well.