100g-150g seems a little more reasonable, I mean, if you're mining for certain minerals its usually for the purpose of upgrading a gun then, which that plus the amount for the gun upgrade itself. I don't expect to go into a mine and earn 200g+ gralats to make a profit off of certain minerals.
But what does need to happen is if the player logs off it automatically warps the player out so that there is no cheating behind this either, cause some players would get the idea that if they keep an account in there they can summon another account of theirs in the same level so they never have to pay again.
And as for entering the level itself have an npc that first ask if the player would like to enter the level so that there is no "I accidentally lost <#>g by hitting this warp".