The fees vary from island to island but are reasonable, i.e. $free to
$15 usually for a 30 to 90 day stay. If you are not a documented boat
(registered only) stay clear of the French islands or at least one or
two of their 'check in' ports where the port captain can choose to
impose a fine. Other cruisers encountered in route will let you know as
you go as to which ones are being PITA's.

I think only the BVI and USVI have moorings that they will charge you
for...about $10 a night and only if you want one...otherwise you'll be
on anchor everywhere; unless you are a marina freak.

Island hopping the Caribbean is pretty easy on boat and crew since
most all passages are daysails, but don't get the idea they are not
'open water' ones.
You and your boat still have to be prepared.

Getting there is a lot of the fun unless you take the 'thorny path'
motorsailing into the prevailing easterlies like so many do(IMHO). A
much safer and pleasant course is to just head east (anywhere above
22N) until you are about 064W and then turn south. This is an 'open
water' passage alright, but much preferred by sailors. Depending on
your boat and point of departure it takes 7 to 10 days compared to as
many weeks bashing into the trades getting to the Virgins (top of the
chain)from the states.