It's really difficult to value a RIB blind because the condition varies so much. It's not like a car where you can get a rough idea just from the age and mileage.
However, the rig you are looking at sounds promising. The boat and engine and both reasonably new, so you stand a reasonable chance! The Yamaha 90 is a great engine, and Caribe RIBs seem to be built reasonably well, although they are not as suitable as some others for any serious offshore work.
Have you read the article about buying second hand in the RIBnet features