I asked the same question about a month back. Apparently, the only deal (for a long time) on a Nintendo DS Lite was during Black Friday on Amazon, for $99. Other than that, the DS Lite pretty much NEVER goes on sale because it doesn't need to.
I just went ahead and bought an Onyx DS Lite from Amazon for retail price $129 because, hey, at least I don't need to pay tax on Amazon and the shipping's free.