Denise,
Here is the list of RSSC tours for the Diamond this month, maybe you can compare prices there.

Diamond Santorini excursions

Just my opinion but Santorini is one island that I think is very doable on your own. My favorite place was Akrotiri, where the ruins are just fascinating. In fact, the town and building are so well preserved, I hesitate to call them ruins.

We did the ship tour that included Akrotiri and the winery. The winery was probably the oddest excursion we have ever been on. After the tour, we were all ushered to a little room in the winery where we all watched a 1/2 hour video of psychedelic images. We all just sat there looking at each other and giggling.

If I had it to do over again, I think I would just taxi to Akrotiri (or even take the ships tour that doesn't include the winery), and spend more time exporing Fira before returning to the ship.