Competition for local business dialtone should eventually drive the price
down. PRI service is technically simpler than multiple BRIs, the cost
to the local loop provider is about as low as you can go, the CPE does
all the hard work.
Livingston should research whether anybody is or will develop a product
to aggregrate a number of BRIs and present them to a PM-3 (or Ascend,etc)
as a 'virtual PRI'. Yes, you can do this now with a micro 5ESS, but
that's expensive overkill.
The actual hardware cost for such a beast would actually be pretty
minimal, perhaps with a street price of $2K or less.
E.g. it could accept 11 BRI circuits and present them as a 22 PRI
B channels, or turn 15 into an E1.
I'd buy one.