To sell a product or service to someone at the same or close to what the product or service will cost you. Hence the person that you are selling to is being given a substantial discount on the normal price. As the term "mates rates" suggests this level of discount is usually only given to mates, or friends, of the person selling the product or service.
Bob: "Hey Johnno, how much did you pay for that computer?"
John: "Got it real cheap mate. Trev sold it to me at mates rates."