I think going anywhere in Greece right now is going to very cheap (relatively) because of the economic/political situation.
The islands that make sense to see depend on where in Greece you are going. If you are say, flying into Athens, Mykonos/Delos is a close ferry away and are well worth seeing if you like ruins. From there you can ferry to Santorini if you like relaxing (or volcanoes).
You should be careful of the economic situation there. If you have a Greek bank card, you will be limited to 60 euro withdrawal limits. Even if you don't have a Greek bank card, in some locations there are reports of ATMs running out of cash. Thus, it might be safe to take a large portion of the money you expect to spend in cash with you ahead of time. Additionally, some smaller businesses are not taking credit cards at this time.
