Well, I put it at such a high amount because it is already attainable to get pvp's if you give up at least 2 FF's, but it's not guaranteed, so the more you shell out, the higher the chances, but it's still by chance. What I'm talking about is a guaranteed pvp, so I guess it's comparable to the potions where you can chance to get 5 health for 3 gems, or you can pay more gems for a potion that you know for sure will get you health.
Also, I don't necessarily think it should be catered to really take sitters into consideration because they choose to sit at whichever level they're at. There are kage-level sitters and jounin-level sitters too. But I guess 10 pvps or so is a more reasonable number perhaps. I hope that explanation makes sense. XD