We don't get involved with costs here because there's way too many variables. Brake jobs are often advertised with a set price for pads and labor, but new rotors, when needed, and other parts are not included. Steel brake lines are not very expensive but replacing them is usually billed on time and material. It's impossible to predetermine the cost because if the lines are rusty, so are the nuts and bolts that attach them to the body. Your mechanic won't know how much brake fluid will be needed. That is affected by the length of the new lines, how much brake fluid runs out while things are apart, and how long it takes to bleed out all of the air. There's a number of steel lines too. Is he going to replace all of them or just the one with a leak? If he only does one line to save you money today, how long will it be before the next one springs a leak? It may be less expensive in the long run to replace both rear lines now.