If you still do not have a Nintendo DS yet but you've been looking at the prices of them for a while, then you will have noticed that the price of them has not dropped much like most other game consoles over time. The Nintendo DS has stayed strong.
The Nintendo DS was released in 2004. Back then it was a great invention to finally have a touch screen game console. More and more games kept (And still are) getting released, and it was no surprise that when the Nintendo DS Lite was released in 2006, it broke sales records. Although the DS Lite was not a completely new console, it was just an expansion, people still wanted to buy it because it was a lot smaller and the screens were a lot brighter. The DSi is set to be released shortly now too which will see even more sales for Nintendo.
You need to be careful when you're comparing prices on the internet for a Nintendo DS. Companies try certain tactics to make it look like they're selling one cheaper when they actually are not. For example, companies may display their prices without Postage and Packaging which will increase the costs, display their prices without tax so you pay a lot more. The reason they do this is because if they add it on at the end, you may just think "I've spent this time ordering it now, I'll buy it anyway" and then they earn more money. You need to check prices in-store because you know thats what you'll pay, and then compare them with the real prices to make sure you do not lose out.