I have $215 worth of gift cards to spend on books.
I’d like to build up my greek/latin/classics/archaeology library a bit.
What are the classic, must-have books?? 
Somehow I’m fed up with all these classic authors. Through all the centuries they had become really exsiccated, dry and lifeless, like mummies. I have some in my shelf, but rarely have the desire to look at them. No more Plato and Aristotle, I swear. I like more books from our times, even if these books deal with ancient dead civilizations. But if you persist, I think Edward Gibbon’s “Decline and Fall of the Roman Empire? falls not in this category. His book is still up-to-date.
How about Apicipius, de Re coquinaria - the only extant, ancient cookbook. You’ll have hours of fun chopping up brains and livers to make sausages, only to find you’ll needs abundances of extinct herbs!
I thought only the sylphium was extinct. There are lots of recipes in Apicius that are quite usable with minor modifications — unless you know where to get sweet raisin wine, say.
For big but useful purchases: The Oxford Classical Dictionary.
Perhaps you’re right - except of course, although it’s not an herb, the garum. Noone really knows what fish it’s supposed to be made of, and noone knows how to prepare it, either.
You’ve given very little to go on, but I’ll nonetheless take a stab. FWIW, I think that annis’ choice for the OCD makes a lot of sense, as you’ll be sure to be using it for many years to come. Otherwise, let’s see what I can come up with…
a) Someone suggested the only extant ancient cookbook, so I’ll suggest the oldest fully extant novel, which is an interesting read except if one expects it to live up to contemporary standards (Callirhoe, by Chariton):
http://www.hup.harvard.edu/catalog/L481.html
b) Other Greek prose:
-If you don’t yet have a copy of Thucydides, now might be a perfect time to purchase it. I only have the Loeb version, so I’m not sure which edition would be best (perhaps someone will chime in). You may want to consider Polybius as well.
- For a great adventure, try Xenophon’s Anabasis.
- If you enjoy reading about the Augustan period, try the relevant books of Dio Cassius (Loeb).
- If you would like to read a continuous ancient account of most of the 5th and 4rth centuries BC (480-302), then consult Diodorus, Bibliotheca, books 11-20 (Loeb). Hardly a must have, but since most contemporary histories of the 5th and 4rth century BC have perished, it is very valuable. Diodorus used sources which we only wish we had today.
c) Someone mentioned Gibbon, but if you would like a modern take on the fall of Rome, here are some ideas:
-http://www.amazon.com/Fall-Roman-Empire-History-Barbarians/dp/0195325419/ref=pd_bbs_1?ie=UTF8&s=books&qid=1198439060&sr=8-1
hth
EDIT: minor change: “oldest extant novel” → “oldest fully extant novel”
This is a tough question without knowing what you already have. As far as reference materials, if you don’t have a copy of Smyth’s Greek Grammar, or Gildersleeve’s Latin Grammar, they would both be good candidates. For dictionaries, the Intermediate Liddell is very handy, as well as Cunliffe’s Homeric Dictionary, which I reference very often, even when reading other authors; the OLD is on sale at Amazon right now, if you’re looking for a big investment.
Will’s suggestion about the OCD is a great idea, and there is a tremendous discount available for the latest edition on Amazon. You might also consider ordering William Harris’ Intelligent Person’s Guide to Latin, which has fantastic explanations for Latin grammar. Eleanor Dickey’s new book on Ancient Greek Scholarship is very affordable and very handy. Rijksbaron’s book on the Greek Verb is also very affordable, and I’m itching to get it myself. If you are interested in Greek History, one of the best single volume histories, in my opinion, is John Fine’s book “The Ancient Greeks”. There is a great deal available on Amazon for “The World’s Writing Systems” by Peter Daniels, although only sensible if you are interested in writing systems. Barbour’s Selections from Herodotus is a great reader, and so is Pharr’s first six books of the Aeneid. And it’s always a good idea to stock up on Cambridge editions and OCTs, as suits your interests.
It’s fun to imagine spending other people’s money on books.
Let us know what you choose, just for fun.
Sorry, klewlis, what are you most interested in?
everything. 
klewlis, I’m not sure if you follow the CLASSICS-L list, but recently a booklist of books offered for sale was posted there. Perhaps this could give you ideas: