As an ex-apartment dweller, I could tell you right away that you don't need to spend any money at all to pay in kind to your not very cooperative neighbors.
The key phrase is "directly underneath me" - I wonder did ever gotten an urge to rearrange the furniture at 3 am? Do you have a jumping rope? Do you fancy a midnight bowling competition in your apartment?
The possibilities to annoy neighbors BELOW are truly endless. Be creative.
If these ideas aren't inspiring enough, you should also ask folks here:
https://www.reddit.com/r/UnethicalLifeProTips/