Children's Orchard has great prices on costumes! I got both of my girls cinderella costumes there for $15 and under each@
Old Navy and Babies R Us/Toys R Us have lots of costumes. Amazon has tons and the Costume Castle stores that open up around Halloween time are usually good too. Even Party City has a ton.
My kids are not 100% decided yet, but I think my son is going to be a pig and my daughter a tiger to represent their Chinese Zodiac years (it was my son's idea).
We usually peice together costumes and purchase needed props. This year, my 8 year is going to be King Arthur (his latest obsession) and my 3 year said he wants to be a doggy (which I have a pattern for. Yay!)
My husband and I usually dress up in the same theme, but haven't really thought that far ahead yet. Last year we were the mad scientist and the bride of Frankenstein.
And let me add one more question...where are good places (aside from Carter's, Costco) to find (inexpensive!) costumes? Thanks!