Most bike rental places charge $4-5 / day. This is very high considering the bikes all cost about $80-90.
Unique Suggestions: It is best to shop around. When quoted price, just ask if "that is their best price".
Fun Alternatives: Best thing to do if you are staying over 2 weeks is to buy a bike from hardware store (San Francsico in Playa Negra) and sell it for 50% of purchace upon leaving. There are no shortage of buyers in fact many just sell to the next tourist.
You can also totally change the life of a local indigenous person by donating the bike to an organization called "The Bridge". They work with local indigenous familes in need and a bike allows a local to get work far from his home. Remember, things like bus fare (one way) can eat up to a whole hours salary for many.
Puerto Viejo and the vacinity is small enough that getting around by bicycle should be ideal. However, many of the rental shops don't maintain their bicycles at all. You can end up with a very rough ride, or even pushing your bike back to town.
Unique Suggestions: Ask the rentor if you can take it for a test ride for a few minutes to check the suspension, brakes, and drive mechanism. The reputable vendors will have no problem with this.
Fun Alternatives: Also enquire as to the procedures if you break down a long way from town. Some of them will have a pickup plan.