Occasionally, big-box stores have weeklong sales on MS points, but those sales are few and far between--though my Target had 1600 point cards for $15 on Black Friday.
Moreover, assuming you're getting them at the normal price, you save no money getting the higher value point cards. The 1600 point card is $20 and the 4000 point card is $50. 800 points = $10. Do the math and you'll see that you save nothing.
However, stores like buy.com or Amazon periodically drop prices pretty insignificantly so you may want to check there. Good hunting!