CompGeeks sells ATX case's regularly for around $23 and shipping is $13. Many times they have a sale with free shipping, or $19 case price. These include a 250w power supply, that will power any Athlon motherboard. To spend $35 for an AT case delivered without power supply is terrible. If you really must have an AT case, egghead.com has them everyday with 250w supply for $20 and shipping is $15.
EDIT Sorry, the cases I listed are only 6 bay mid towers. Not big enough for use with an old AT motherboard that still might be using old full height drives with there 486..