I was talking to a retailer last week about a considerable discount they were receiving form a supplier. They were pleased with what they had negotiated. On checking their invoices and comparing them with others it turned out that the base price off of which their discount was calculated was higher than what appeared to be usual.
The extraordinary discount negotiated disappeared once you took into account the inflated base price.
If you are offered a discount, make sure it is off the standard wholesale price quoted to all other customers.