Unfortunately, you don't have many options.
Occasionally, during a XBL deal of the week, they will put an arcade game on sale.
You used to be able to buy them cheaper from a couple eBayers, but that's gone now.
When Amazon started selling them, they had a couple deals, but haven't had a single one since.
Your best bet is to buy a points card on sale. Only problem is, you don't have many options for that either. A lot of cheap points cards were floating around eBay, but like the arcade games, have disappeared. The last deal was at Target a couple weeks back (buy 1600 points, get a 5.00 gift card). Back in the day, there used to be some ridiculous deals. I still remember when Circuit City had buy one get one. Shrike picked me up 2 cards, and then CC let me do the deal a few days after it was over since they never restocked during the deal. Getting 80.00 in points for 40.00 was
If only I knew what things would have been like now, I would have picked up 200.00 in points, easily.