Watch for specials online. slickdeals.net and dealsofamerica.com are good places to watch for SD card specials. I posted a deal on the photography boards here a couple of weeks ago that was ridiculously cheap, 4 Kingston 1GB cards for around $15 after $10 google checkout and rebate (I think it was a buy.com).
I only buy SanDisk and PNY, my last 1GB PNY card I bought online cost me $5 (I think I bought it from buy.com and had $10 off with google checkout, no rebate). Kingston is another brand I see recommended often.
Be careful buying SD cards from ebay, I've read several posts on other boards by people who have had problems with cards bought from ebay. Don't remember the specifics though, sorry.
My one other tip...buy more than one card, memory is cheap these days and if you have a problem with your one and only memory card while you're on vacation you don't want to be searching for a memory card especially if you're at a theme park, that's missed picture opportunities. I usually bring several 1 and 2GB cards with me on vacation, some standard, some ultra (for taking video with my camera).