One of the great things about MS points is that to a degree they decouple real world currency from the service, allowing you to blithely spend money without a connection to real world dollars and cents (there is also something about creating an international currency but I like conspiracy theories too much). If you want to break down that barrier you can use this online calculator
to figure out how much that GRAW content pack costs in real dollars (yeah, it's still a lot).
Thanks to Major Nelson
for the link.