I rented just the mask and snorkel at Cruz Bay Watersports up by the Lumberyard. To be honest I can't remember what the cost was, but I know that it was very reasonable. I want to say it was about $8 per day... My hubby rented the whole set and again not sure what it was but it was reasonable. if you need to you can rent just fins too.
We just got back from STJ and rented mask,snorkle and fins from Cruz Bay and it cost us $10 per day for the set and $15 for a set with a prescription(magnification)mask. The RX mask worked surprisingly well considering I wear progressive lenses in my glasses. Everything was crystal clear.
We took our own masks and snorkels, but rented fins from Low Key. It was $4.00 per day per pair. You might want to invest in liners of your own since you might not get the greatest fit from the rental fins.