As an option, to rank items, you can use the amount of money that the item has earned instead of just the number of upvotes. You can even replace upvotes number in the UI as well. This way frequent meme-upvoters will have less sway over more picky users. It'd be easier to register astroturfing accounts and to promote something to the top, though.

One issue is that the amount per upvote is only known at the end of the month - only then you know how many upvoted to split the $1 over.

You could keep updating the weight you give to each upvote. That would mean upvotes you make at the start of each month are valued too highly before getting valued less, but as long as the creation date of all the accounts are distributed evenly across the month that should even out.

Alternatively you could base it on the number of upvotes a user gave last month, before correcting it at the end of the month, although that system might be easy to game.