Game had in a second hand ds the last time I went in. It was around £85 with a secondhand game and came with a warranty. They're very good at that kind of deal, apparently, so that may save you a little chunk of money.
I bought dd a new ds lite, I have to confess, but have bought her second hand games from Ebay (eg Tamagotchi game, not the latest, but a proper ds game nonetheless, for £4.50...she knows she's getting it and is really excited about it). They seem to be increasing in price though, but there's a lot of weeks between now and Christmas. I'm biding my time for an Animal Crossing game at the moment
In terms of the Star Wars stuff, I suspect it'll be a case of just shopping around for the best deal you can find.
I really feel for you. There's nothing worse than feeling that you can't give your kids just what they want at Christmas, especially if they're not greedy. My mum had 6 kids and not much money and she always went into debt for Christmas. She wasn't a very loving mum, but I always got the feeling that she cared about me if I got the gift I wanted for Christmas (selfish 6 year old head speaking there, not the grown up version of me, obviously .
I'm not advocating that you go into debt to buy gifts for your kids, but I can understand your dilemma. It makes it harder that your little ones are so sweet and innocent with the santa situation. I always told my dd that we had to buy the gifts, then we send them to santa and he decides whether to bring them back! That way we get away with the 'it costs too much money' scenario.
I hope you manage to find a reasonable solution and grab a bargain or two that'll keep you and the kids happy.