Technically, after the Skype call is initialized, the member is charged in the first second with the "$" set by the model and afterwards, at every 60 seconds.
Example:
Your price is $3 per minute:
You answer a member's call, in the first second of the call (time 0:01) your account is credited with $3.
After 61 seconds (time 1:01) your account is credited with another $3.
if the total call time is 3:59 sec, the cost is 4min x $3 = $12
if the total call time is 4:00 sec, the cost is 5min x $3 = $15
if the total call time is 4:01 sec, the cost is 5min x $3 = $15
if the total call time is 4:23 sec, the cost is 5min x $3 = $15
if the total call time is 4:59 sec, the cost is 5min x $3 = $15