Here's an article on printing costs for the PRO-100 --
Cost of Inkjet Printing Report Canon PRO-100
The rebate Canon provides as others have said is an American Express gift card, which you can use at many retail stores including Walmart. Some banks may give you cash for the card. There is a small fee that gets deducted from the card if you don't spend the entire sum within a few months of receiving it. I don't recall the specifics of this fee, but it's not very much. Canon is quick to send the rebate, especially if you pre-file the rebate forms on-line, but you still need to send in the receipt, bar code, etc.
If you already have a PRO-100, buying another one with this offer is less than half the price of a full set of ink cartridges, which is about $110. And you also get photo paper and a spare print head. Canon doesn't seem to mind if you buy another printer as long as it's not during the same rebate cycle (at least they haven't in the past).
Precision Colors sells the necessary supplies for refilling the OEM ink cartridges if you're interested in going that route.
B&H also is selling the PRO-10 and PRO-1 for $250 and $450 respectively if you use the code mentioned by Adam. Both use pigment ink, instead of the dye ink in the PRO-100.