Any drive, no matter how good, can and will eventually fail. The best thing you can do is back-up regularly if what you are storing on there is important to you.
I'm among many professionals who rely upon hard drives to hold and deliver large amounts of stored data, at high speeds. A couple of recommendations for reliable drives that come up regularly among my peers are Hitachi Ultrastar Enterprise Class, and WD Black Caviar. These are the raw drives - you'd need to provide your own housing, which is pretty simple.
I'd also suggest a RAID Level 1 (basic mirrored drives), which are becoming more and more affordable. This would provide slots for two or more drives and automatically provide a mirrored copy of the original. If either fails you just slide in a new drive and the mirrored state will be restored in a matter of time.
If your budget is $100, it's anybody's bet which drive will server you better than any other. Larger drives with active cooling generally do better than smaller drives with passive cooling. Again, any of them can and will eventually fail. I've had good luck with the basic Lacie drive, and with Iomega Prestige - both in your budget, and both work with Macs (I only use Macs).
