Display measure based on selected parameter value

Hey guys,

Consider the sample table below: 

I have a card on my dashboard where i want to display a measure = cost/quantity. However, i want it to have an option where the user can select either to view it in GBP or in the local currency so i created a field parameter and chose the 'GBP Currency' and 'Local Currency' as the values.

So, the goal is to have the user select either the GBP currency or the local currency and have the measure calculated accordingle. So if he selects gbp currency, the measure would be calculated as sum(GBP Cost)/sum(Quantity) and if he selects Local Currency the measure would be sum(Local Currency Cost)/sum(Quantity).

Thanks in advance for any tips or help!