I have been living in PR for over a year and an half and will share any information I have for those interested in living here. We are military, so our situation is a bit different in terms of moving expenses, taxes, etc., but I can comment on general cost of living, etc.
Housing here is a bit cheaper compared to the States, depending on where you are looking. We live on the NW area of the island, and you can find housing in any price range. You can find cheap studio apt. near the beach, weekly rentals, monthly, beach houses, etc. The best place to look for housing is http://www.clasificadosonline.com/
This is bigger than craigslist here, although that too is catching on. Places by the beach can be affordable on this side of the island, which is different from the States, in a great way! As for San Juan, I am sure it is more expensive in terms of everything then over here on our side.
Groceries are a big problem here. It can be difficult to find all you are use to in the States, especially produce. You would think that a tropical island would have amazing fruits and veggies, but it can be hard to find a ripe tomato on this island. You can find tropical fruit on the side of the rode, to buy or pick! Pinas (pineapples) are amazing here, as are the mangos. Groceries are more expensive, especially frozen food. You get use to the grocery stores here, but it is a big adjustment. When I go back stateside, I literally stare in amazement at produce sections!
Any other questions? I would be happy to answer them for you.