I have the sprint unlimited data plan thing with a novatel u727 usb modem, and it ends up being around $63/month after all the fees and such. I couldn't find anything on lowering the cost either, but if I hear anything I'll post back here.
I'm going to be working to get mine working with the n810 shortly, but as mentioned previously it'll probably require a battery powered hub or some sort of external power hack.
Best of luck.