It all depends on what you want to do with a scanner. You have four choices: SCSI, Firewire, USB & Parallel. Don't even consider parallel. The speed is pretty bad. Firewire is going to break your budget. SCSI is good but USB probably gets the most bang for the budget. You should look carefully at the software bundles that come with each scanner. They can make or break it. I personally have a Microtek X6EL SCSI scanner. I bought it a couple of years ago. For $300, it came with a full version of Photoshop 5.0. Worth the investment.
Here is a link to ZDNET's shopper guide for scanners.