I would imagine that they really don't need to drop the price of the DSlite. They've already discontinued the DSlite, so it will sell out to people who want to retain GBA compatibility and it will just be gone eventually. $99.99 seems fair enough as it's final price until they are gone.
The regular DSi is already cheaper than the 3DS, so it will sell down to nothing eventually too. Didn't they discontinue the standard DSi also? I'm not sure about that, but I believe they did. A quick search on that was unclear. Still, I think a $129.99 price point would be more practical for the standard DSi.
The DSi XL on the other hand, well that's basically their custom-deluxe DS. I think the price of the XL being the same as the new 3DS price is OK since the XL has it's own unique appeal with those giant screens. Still, it IS an older system now compared to the 3DS. I think a $149.99 price would be more fair.
Given that the 3DS isn't selling as well as they hoped, it would be in their best interest (I think) to keep both DSi's around for a little longer than whatever might be in their plans right now until the 3DS sales pick up. A slight price drop to the DSi line could really help their bottom line.
If I was Nintendo, I would continue to make DSlite's in special edition form for awhile longer. There are people who like the GBA compatibility and the fact that it plays more than just one generation of Ninty's games. And fans really eat up those special edition releases. There's still money to be made there, I think.
With the DSi....a slight price drop, as I suggested above, might stimulate sales a bit. Possibly, they should extend the DS/DSi development for another year (beyond their current plans) to give the 3DS time to build its user-base.
I guess it would depend on how much profit they are currently making on DSi hardware right now as to how practical it would be for them to drop the price. But I still think that if they released special edition DSlites every now and then, that would be a successful sales tactic as the DSlite IS their biggest selling handheld system of all time and people do still want them.
It's hard to anticipate what Nintendo will do. They like to say "no" or "no comment" or that they "have no plans" for this-or-that when asked direct questions until THEY are ready to announce such things and make it sound like it was their idea.