Well, I perhaps have more flexibility than others, but I have been able to find pretty good deals on most of my Greek texts. There is much more than just amazon. You need to take advantage of the different amazons, amazon.fr being a fair bit different from amazon.co.uk. And there are different abebooks (abebooks.com, abebooks.co.uk, abebooks.de, etc), and abebooks is better overall than amazon. Then there is alibris.com, antikvariat.se, ebay.it, ebay.fr, etc, etc. And there are big disparities.
For example, Homer Opera IV new for $50:http://www.oup.com/us/catalog/general/? ... =relevancy
But new on abebooks.fr for 18EUR with free shipping to UK:http://www.abebooks.fr/servlet/SearchRe ... =t&x=0&y=0
What I suggest is:
1) Figure out early what books you will need,
2) Figure out how flexible you are with respect to editions,
3) Get the ISBN numbers,
4) Search all the sites you can think of. And save those searches right after you do them on your menu bar in a folder browser depending.
5) You can then run those searches every so often in a matter of seconds.
6) You can score some sweet deals that way. If you want the Hornblower commentaries for Thucydides, you will have to pay. But recently I got a Weil Polybius VI (Les Belles Lettres, French) ex-library (somewhere in Wales) hardcover, probably never checked out, great condition for about $10, where new would have been closer to $50, and it is arguably the best version available, only competitor being a much older Teubner.
It is supply and demand, and there is not great demand for classics texts especially if it happens to be a Teubner in South Carolina (hint, hint ).
And hell, sometimes you can just print out a nice GFS Porson fonted version from the web and console yourself that you have nice big beautiful font to read and big margins to fill with all your profound insights. Actually, I am rather horrified by the smallish fonts that seem to be used in just about all Greek texts, and I may just switch to printouts and ring binders in the not too distant future; I am not happy when I can't make out the breathing mark under a circumflex. (Does anybody else have this problem??)
I'm not a student, so I'm not sure how helpful this is to all of you, but I more or less refuse to spend much on a Greek book anymore for the simple reason that if a book costs $30, I can probably find three that are just as good for $10, I'll read the three Demosthenes now, wait on the one Xenophon. The texts are thousands of years old, the best scholarship was done over 100 years ago, we are well into the public domain for the most part, I would try to avoid blowing money on Greek texts unless it is some version you absolutely have to have.