Our clients at www.racksale.com buy bw at 100gb increments.
Price is $50/100gb
We have many 'adultclients' that like that setup.

Our sistersite at www.adulttech.net has a more flexible setup, where you pay 0.7 pr gb used.

If you really want flexibility you should checkout
www.pornhost.dk/auction - there you can buy bw at auction..its working out pretty cool...
last night a lucky guy bought 50gigs at $2

take a look at it or write me at john @ racksale.com