We have updated on how we handle usage charges for our users, since some users might use one set of functions and others will use different functions. We have implemented a credit based system. Each action uses up some number of credits, by subscribing to a plan you will get N monthly credits (you can always subscribe to additional plans).

Current list of actions that use up credits (you can always check on it here, that page also contains your usage of credits):
Check price and stock of an item - 1 credit
Check price and stock of "compare to" item - 1 credit
Instant price check (when adding an item) - 1 credit
Reprice a single item at Ebay - 1 credit
Reprice a single item at Amazon - 1 credit

Current subscription plans available (see them here as well):
$29.99 per month - 250k credits
$119.99 per month - 1.25mln credits (20% discount)
$209.99 per month - 2.5mln credits (30% discount)

An example on how credits are calculated: you have 1,000 items and check them every 3 hours - 8 times a day that is (default $29.99 plan that most users are subscribed to):
8*30*1000 = 240,000 credits used up monthly