I don't know how many you buy, but we get collage prints from Wal-mart. It's about half the price of the larger photo cards (4x8 or 5x7), if you get the 4x6 (or less, if you plan ahead and don't do one-hour!). We also like the 4x6 because you can put it in a regular frame or photo album for people who like to keep their photos. It's not quite as fancy as some other cards, but we like it and have made some cute cards the past couple of years. We write a letter to send with it.
I'm sure you could use photo programs to make your own collages (I've used Digital Image from MS); but most printers would go through enough ink that it's probably more cost effective to print them elsewhere.
Also, snapfish and walmart use the same software, so you can also do this at snapfish, prepay for prints (which can be as low as 8 cents, though you still pay shipping unless you get a shipping deal) and get a really good deal! I have yet to plan that far ahead with my Christmas cards, but maybe some day . . .
Have fun:)