Have you ever been here? I guess not, because otherwise you would not think of using a bike as your means of transport.
The infrastructure is absolutely not bike-friendly, neither is the weather (hot & humid).
Traffic is, ehm let me find a friendly description, hectic? I guess that's the main reason that you hardly see people using bicycles.
Anyway, flat land is anywhere around the coast; as soon as you go more inland there are hills.
If you're looking for cheap places to live you're in bad luck if "flat" is important; the cheapest houses/apartments are found in the interior. Anywhere close to the coast is more expensive with the highest prices in the San Juan metropolitan area.
Now, in the metro area there is public transport, so if you would want to live without a car that could be the place to be for you.