Some key points when you consider costing for a call center includes:
1- With increase in the number of seats the per seat cost for a solution would generally decrease. The obvious difference comes with the addition of 10 seats.
2- Maximum ultility of the seat would reduce the per hour cost of operations for example if you are running a 24*7*365 operations the cost would be much lower as compared to the an 8-10 hours shift a day. Reason is the fixed cost incured like
ii) licensing fee (in case of hosted solution)
iii)ultility bills etc