I currently have a 750GB 5400 RPM laptop HDD and it's just slow, so I want to upgrade to a 250GB 7200 RPM HDD for $50 or less. I'm going to turn the 750GB HDD I currently have into a media HDD to store videos and such.
Newegg would have what you're looking for. I've seen them as low as $59.99. Of course they also have higher storage (500-700gb) for about $20+. Some items even have free shipping.