The two Islands offer different value for the money.
On Santorini, you pay for the view, even a dump with a view is expensive. So you can look around find an array of places, all with a similar view but different prices, see what fits your bill. Of course, consider how much you will be able to see that view, if you are out busy, and only there in the dark after a long day, no time to sit for a few hours and enjoy the view, then scale back and spend the money on a nice restaurant with a view.
On Mykonos, it is less about the view (Well, the view is probably still better than out your window at home) and more about luxury. Want a veranda to spend the hours?, a pool? Big Room?, a Villa? Again, consider how you spend your time. If you do the lay on the beach and then party all night, probably no need for luxury, but a quiet, comfortable room. If you are planning lots of beach, then definitely look at places near or on a beach, which will run higher, but a bargain in October. Santorini will likely not see bargains yet in October.