Pro Features Lab Fees are rounded up to the Nearest Penny, which could cause some confusion in your calculation of 2 cents + 5% per complete.
This is because TurkPrime charges by the HIT completed as they come in (the completes). So each complete is calculated on its own (this is because a user will sometimes want to stop a study short or there is some worker submission issue that causes the study to complete before the requested amount reaches it's goal).
For example, if you were paying workers 50 cents, it would be 5%*$.50 which would be $.025. However, since we cannot charge for fractions of pennies, this will get rounded up to the nearest penny and be $.03. If you were looking to collect 100 responses, the pro feature cost would therefore be (.03*100) + (.02*100) = $5.
However, if you calculate the formula before rounding, your calculation would come out to a charge of $4.50 (.025*100 + .02*100).