$1 for a good review, $10 for a bad review
So, a bad review, which can ruin someone's business, is worth more than a positive review that should praise a service.
The purpose of bad review is to defame a genuine service and that's not acceptable from ethical point of view. OP, we don't do such things in order to earn money, even if you pay $50 for a bad review still most of us won't accept such offers because it's like selling one's soul for money.