Yeah, you should list your budget and possible applications you want to run.
If you're going to use the 2 bay NAS in raid 1, I'll let you that RAID is not a backup method.
Instead, I'd recommend a 1 bay NAS and an eSata external drive attached for backup purposes. For instance, a Synology DS111 and this eSata Macally enclosure and two Samsung 2TB drives or two Hitachi 3TB drives
In this case (just roughly estimating, you can try shopping around for better prices).
2TB solution ~ $400
3TB solution ~ $550
Otherwise utilize the 2 bay NAS in raid 0 and attach a similar sized external solution for backup.
*edit* my fault, didn't see your $100 budget, you still should consider a backup to the drive though.
With $100 budget, if you're transferring large files around (hundreds of MBs to GBs) then you'll see pitiful speeds.
If you're willing to put up with that performance, then how about attaching a portable 2.5" hard drive to your router if you have that functionality?