There is probably no more volatile marketplace at present than that
for telecommunications services -- in many sectors (eg mobile
communications), competition has resulted in large price
reductions. This has, in general, not been observed in the "fixed
service" market, where established telco monopolies such as Telstra
in Australia are able to maintain comparatively high prices.
In general, PSTN and ISDN services are charged in a similar
way:
Installation (or connection) cost, usually between $200 and
$500 per service in Australia, depending on location, etc.
Annual (or monthly) "access" charge. Typically between $15 and
$70 per month in Australia, depending on selected "plan".
Call costs, at widely varying rates depending on location,
distance, "plan", etc.
In Australia, a strange disturbance in the political "force"
means that local PSTN calls continue to be charged at a flat
(per-call) rate, typically 25cents. This will
change. Maybe... For ISDN, most calls are time charged, with
differential rates for data and telephony calls -- the recent
introduction of flat-rate voice calls on Telstra's new "Home
Highway" ISDN service being an exception.