C3PX said:

...they also make them an odd price and only sell msp in even numbers (400, 800, 1200, etc. And most games you can purchase on arcade and most DLC are priced to match these numbers, except for L4D's. In order to dish out the 540 msp (I think that is how much they are) you have to buy 800 msp ($10) leaving an orphaned 260 points you didn't even want to spend money on that are now just floating around on your account, tempting you to spend it on crap you don't need... like eye patches for your avatar, or Lando's administrative outfit.

http://www.pvponline.com/2009/03/12/match-points/