I'm using SPSS to analyse some commercial data. The data is organised with a row/case for every item that can be sold (a SKU) with the sales quantity and retail unit price as variables. I am trying to calculate quantiles of the retail price of units sold. This is not the same as quantiles of the variable 'RetailUnitPrice' as some SKUs have sold many units. For example, if I have two items - A and B which cost £1 and £1001, and I sell 3 of A and 1 of B, the median retail unit price is £1 (since this is halfway between the second and third items sold when ranked by retail unit price) but SPSS would give me a median value of £501 since this is the midpoint of £1 and £1000.

Can anybody help? It feels like I need to transform the data in some way and then run descriptives on a new, calculated variable, but I'm new to the software. Thanks!