Yeah, bandwidth is expensive. I pay about 8 bucks for my webspace. I've got one gig of space and, (I think) a 5 gig per day bandwidth limit. Lets just assume that 8 bucks buys 5 gig of transfer.
Now say T:V beta is about 2.5 gig.
Thats $8 for two people to download at that rate.
500 people in beta $2000 in bandwidth from downloads alone (and that assumes no additional downloads for patches).
Staff, say 1 person at $10 an hour to read and catagorize bug reports etc. $400. For a week. Every week. at 5 weeks, it's $1000.
Beta website, (small cost, but necessary) for beta forums and other stuff.
Extreme example, but stuff like that adds up quick.
And Thrax's example doesn't take into account stuff like ongoing development costs (Programmers, etc, fixing the bugs...). You can see why Beta "playing" isn't a good thing for the game. Beta "testing," on the other hand, is what should be going on.