One hundred pots mixed grain — that’s what the retailer paid (4,5)
I believe the answer is:
cost price
'what the retailer paid' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'one hundred pots mixed grain' is the wordplay.
'one hundred' becomes 'c' (Roman numeral).
'mixed' is an anagram indicator.
'grain' becomes 'rice' (rice is a kind of grain).
'pots' with letters rearranged gives 'ostp'.
'c'+'ostp'+'rice'='COST PRICE'
'that's' is the link.
Can you help me to learn more?
(Other definitions for cost price that I've seen before include "Outlay required from a retailer to buy goods for resale" , "What the seller pays for goods" , "Expense of production of goods, no profit added" , "Amount before retail mark-up" .)