What do you want to do with it. Considering PDAs these days are quite versatile. I have a Palm treo 650 which is sim free that I got on ebay for £110 plus postage. It is a smart phone, so a phone with PDA functions. Palm OS is considered the best OS of all PDA formats. Friends of mine have windows mobile powered units. One guy was showing me his one for the first time running Windows and it crashed when adding my number, so was not exactly what I would call reliable. Look for big names. Smaller companies tend to have no support for users and just dont do the business.
My 650 was a bargain as they are about £150 usually. However you can certainly get a Treo 600 (just as good but without bluetooth) with a 1gb memory card and maybe a carry case for your money. Do check the battery life of any you buy second hand as they do degrade and try to get a case for it. There are plenty of bargains to be had.