Well, believe it or not, the block cost nothing.
Check out:
[link|http://www.dsl.net/homepage/cost.html|http://www.dsl.net/homepage/cost.html]
We're getting the "NETgain1500" plan, and we just requested a block of 100. Like the flyer says, they ask you why you need them, etc..., but in our case it was easily justified as we're hosting.
-Jason