Question about prescription charges?

Hi - hoping someone can help! My parents-in-law are looking to move out to Tenerife next year. They understand the S1 form for accessing healthcare but there is one thing we just can't get a simple answer on. We know they will have to pay 10% of the prescription cost for any medicines, but is that 10% of the basic prescription cost or 10% of the medicine cost? Eg if their medication is expensive we need to know if they need to budget for 10% of that expense (and also check what that medicine costs in Spain!) or if it's just 10% of the simple prescription cost (which I think is just under 12 Euros?). 

Thanks!