I had a $50 cert last july for my bday, I picked up 2 50cd spindles, a cd repair kit, and half of half-life (it was $20 then, and IIRC I only had like $10 left on my certificate).
BTW, they can't be cashed AFAIK.
If you buy something with them, the change is given in the form of a new certificate.
So essentially you are stuck with spending the whole thing there. If you really want to cash it out, buy something that you can ebay or sell elsewhere at a close price, you may be able to get most of your money back.