Well i like the concept, for me it seems like a tower defense where you make the path, but, as others point, the game is really unbalanced, it takes away all the fun and the extra mechanics, you really don't need the monsters at all to get money.
What would I do instead, well, as it seems there are diferent monsters and visitors, i would allow only certain players to be abducted for some monsters, and they should give a better amount of money instead of giving so little, in fact, the only way the monsters have a certain value in game is that the tips were the only source of income, better monsters, better income.
As you might see by now, there are hundreds of ways to make this game better, more enjoyable and perhaps adictive, it's a wonderful concept, but need more balance.
Good luck!