(one) can't say fairer than that

One cannot offer or propose a better deal. This offer includes a 20 percent discount, and I've waived all the fees. I can't say fairer than that. I'll even drive you there. You can't say fairer than that, can you? Yeah, it's an old car, but it still runs pretty well and Aunt Sue would give it to you for free—you can't say fairer than that.

you can’t say ˈfairer (than ˈthat)

(British English, spoken) used to say that you think the offer you are making is reasonable or generous: Look, I’ll give you £100 for it. You can’t say fairer than that.