Hi Chuck!
Several grumblers, including myself, have the Dell 3100CN (the smaller cousin of yours) and love them. When the 3100CN goes on sale, it costs about the same as buying the toner (which is included free) We purchased an extra set of toner with ours, more than a year ago, but are still using the original ones that came with it. It lasts a long time. We use it for mailings and color postcards.
If your 5100cn is new, it should include 4 toners with it (one for each color). They don't ship with the toner's already installed in the machine, but they should be in the box.
Dell printers are actually Lexmark/IBM printers, rebadged. However, the same model in a Lexmark name takes a slightly different toner cartridge design. I believe Lexmark sells the same printer engine as a Lexmark C762n, but they have different cases and software. There are minor differences to make sure you buy the consumables from Dell.
I see them on EBAY for $249 (for all 4), but this is for a remanufactured(refilled) product. Refilled laser printer toners are almost always leaky, streaky, and not up to the same quality as new ones. (especially the type that contain a drum) Approach with caution.
I also see Ebay auctions for (sealed) ones for $319, $359, and $399 "buy it now" price. This may be a good alternative source, if the seller has a positive feedback rating.
You'll also notice that Dell has different prices in each of their sub companies (home, small business, corporate, educational, govt, etc) and each one has sales every other week. Consumables often go on sale for up to 30% off.
One thing is certain, you'll love the printer. They have a built in ethernet print server that your whole office can use, and are workhorses.
Good luck
Mike