Seems like an odd amount. Too low to amount to much in total, but an annoyance to the user. Perhaps they figure the PITA factor will discourage frivolous downloads but won't dissuade anyone who is serious.
Or, perhaps they are moving the market toward accepting micropayments. ;)
I think i's just to sort out the serious beta users vs "let me download it and not use it at all." I doubt MSFT make smoney on this at all by the time they pay bandwidth, credit card fees, accounting, customer service etc.
Maybe another more sneaky reason is to insure that the beta was being downloaded by UNIQUE users. A $1.50 might be enough to stop someone from installing this on 7 of their computers sitting around the various rooms of their house which may not serve the interest of the beta program. With the type of network infrastructure that microsoft has in place, it shouldn't cost that much for them to serve.