Right now it costs 70% of the upvote value. However, we'll add the discount soon which is currently about 30% max. The discount gets calculated of the account value of the user.

So by this we give cheaper upvotes to people who don't have a lot of money, but in return vote sellers (and us) receives less.

Yes you could upvote yourself and friends. But selling votes is still more profitable than curation alone.