What would the core charge be on a 20 dollar water pump

I don't believe they add a core charge for pumps. I believe core charges are for items like batteries where the seller is held accountable for proper disposal/recycling or sir charge toward disposal/recycling sites by local/state/federal laws. Pumps aren't generally made with hazardous materials.

I believe their water pumps don't carry cores and are all new. But if it did, it depends on how badly the manufacturer wants a core back... Typically no more then $10