Margarita island is pretty large for an offshore island, and due to the distances between towns, it may prove to be more difficult to get around than one may expect. The paved main roads are in excellent condition, but some of the side roads offer a less smooth ride, as the asphalt may either be cracked or cease to exist in some sections.
As the general rule of thumb, public transportation is the cheapest way to get from one town to another. If you speak and understand Spanish, this should be relatively simple, but if you don't know the basic phrases, this could definitely become a major undertaking.
If you're an adventurous traveller, I would recommend giving the public buses a try, as it would be quite the experience. The buses are quite old, but functional and the ridiculous fares (less than US $0.25) makes it an unbeatable deal.
Much like neighbouring Trinidad, there are certain vehicles which ply certain routes, somewhat like taxis operating as buses. In Margarita island, such means of transport is known as 'por puesto'. These vehicles remind me of automobiles wich are normally found in antique shows, but the ride which they provide is very comfortable.
While this method of transportation is about $1, and nomally faster than the bus, unless you're paying for all the seats, you must wait until the vehicle is full before it will move off.
Regular taxis which will take you anywhere you need to go, are usually used by tourists and so therefore the costs will reflect this. I paid anything upwards of $15 USD for a private taxi to take me around a few times, and depending on the distance being travelled, the price fluctuates greatly. While this is obviously the most expensive way to get around, it's also the quickest and most convenient.
In short, regardless of the depth of your travel pockets, it's definitely possible to get to just about anywhere on Margarita Island.