Explain the cost of PC to a novice!

Anything between 0.01 and 19% of gross weekly profits for level 1 PC.

Click on "About Us" then click on "Betfair Charges" and scroll down to "Premium Charge Summary"

it tops up the commission you pay to a specified percentage of your gross profit.  If the percentage you pay already exceeds that percentage you won't pay anything.  There are 4 different rates that can be applied depending on your lifetime betting stats, 20%, 40%, 50%, 60%.

If you really are a novice my advice is to not even think about it until you get near to paying it.  Betfair tinker with the charge structure quite often so just concentrate on making as much as you can with whatever charges apply to you at the time and if your charges change, review your strategy.