The temptation maybe there to get a boat load full of cheap SD cards instead of one good SD card, but don't!
They're more expensive for a reason. Some are faster than others (Lexar 600X for example is faster than a 200X Lexar card, and more expensive) but the main and most important thing is RELIABILITY!
The new San Disk Pro cards have a crazy low failure rate, and a lifetime warranty! These more expensive cards will last longer, and in more extreme conditions. Cards dying is frustrating. What's more frustrating is losing images. Is an extra £15 for a professional card too much to pay compared to the possibility of the cheap card failing with 300 images and a day of shooting lost. Forever. I think not.