There are good deals over at Ebay quite often for both iDs and iD Ls. I've noticed iDs are a little more rare and can range anywhere from $30 for used ones to $100 or more for brand new ones. I've seen new ones sell for less though. I've sold one of my new ones for about $50. It depends on how patient you are and whether you're willing to wait on good deals to come around. iD Ls are more plentiful on Ebay and are going for as little as $40 for new ones.
I have both and though the iD has it's own charm (it was the first color Tamagotchi to introduce the IR download concept after all) I personally prefer the iD L series. It has a lot more content, characters, and locations. If you can swing for a 15th anniversary iD L, there are cool characters, including ones you see on the vintage Tamagotchis, and there are locations that you can eventually unlock that pay homage to some of the other Tamagotchi versions.