For different service levels, for example, 90% of calls handled in less than 20 seconds, what percentage of price reduction can I expect? Also, what is a reasonable increase in utilization percentage?
You should run Erlang models of the current and possible service levels for your workload -- that will give you the number of agents required and the occupancy. You can calculate cost based on the number of agents required. If 90/20 is your current service level and you are a small to medium-sized call center, you should have a significant opportunity to create some savings, as that is a high service level that results in relatively high cost and low utilization.