Is it less expensive to purchase tickets from the US well in advance (as it is with most air travel here)or to purchase tickets while in Greece or Turkey? If it's better to buy in Greece or Turkey, how far in advance of traveling?
For travel in the Greek islands it's best to purchase your tickets when the saver fares go on sale, which has come and gone already for the upcoming season. There may be some left, but not many. Check the Aegean Airlines website. I once bought a very inexpensive last-minute plane ticket from Bodrum to Istanbul but it was in May before the summer rush. I do believe that high-season tickets should be purchased in advance. Start looking on the airfare websites about 4 months ahead of your need for the best prices
Last year I bought tickets for travel within Turkey from a travel agent in Istanbul over the internet. The savings was only 5 euro per ticket. The advantage for me was that I could put the tickets on hold and pay for it at the last minute since I was trying to take care of some other logistics and didn't want to lose my money in case I decided to change the destination.
If your dates are set, then just as easy buying over the internet. If flying in Turkey, do keep in mind that a lot of the low cost carriers use SAW and not IST.
You may have a little problem booking in advance online. Last I checked, Onur Air still did not have its English language website up. So, unless you have at least some Turkish, you may find online booking problematic.
Thanks for all the suggestions. My travel dates are at the end of June and first of July. I'll start checking things out now.