Its a suggestion. In this sentence they are suggesting that the price be 55 dollars. You might also see or hear "lets say." It is the same thing, and it basically suggesting, why dont we both agree (or say, or lets say) on this. An example of how it might go in a conversation. Buyer: "How much are you selling that item for." Seller: "Wells its an antique...lets say 55 dollars?" Buyer: "Sure that sounds like a good price." OR "That seems too expensive...why don't we say.. 50 dollars?"

It means "approximately". We use "say" when you have to estimate the value of something or assess a situation spontaneously and you don't want to be too precise.