If you're looking for new equipment, pick up a copy of Computer Shopper. This is the "bible" for buying new equipment. Skim through it for the best prices and give these distributers a call. In most cases, the advertisements must be placed months in advance; the actual price may be even lower than the advertised price! Two other things to note are the warranty, return policy and location of the company (companies within the same state as you may be required to add extra sales taxes).
If you're looking for the expected price of used equipment, then scan the newsgroup misc.forsale.computers.pc-clone for similar items. This will give you the best idea as what to expect. Don't make assumptions that the price of used equipment will follow the market trends of new equipment. For instance, when new memory prices nearly doubled, the used prices were barely effected.