How much would it cost for a complete new brake system?

It would be for a 1970 Ford Mustang.
This is the Craigslist ad:
1970 Mustang Coupe Project Car, 302, Automatic, Factory AC car, Magnum 500's.
Newer:
Seat Covers
Carpet
Gas Tank
Flowmaster Exhaust

Engine runs good. Not driveable - no brakes, will need to trailer or flatbed.
86,000 miles

Also how much do you think I should pay? (He is asking $3400)

ENVY YOU! Check the unibody for RUST and damages FIRST MECHANICALS are easy to fix BODY WORK and paint or NOT easy to fix! STEERING sucked on those year cars! New brakes are on INTERNET from BRAKE WAREHOUSE and more places! Have NEW brake lines and hoses installed and perhaps even a new MASTER CYLINDER! CALIPERS should clean up with SAND BLASTING! Expect about $500 or so if someone ELSE does the work! BYE NOW!

Brakes not working is a very vague term. A worst case scenario requiring new lines, hoses, pads, shoes, drums, discs, calipers, booster, pistons, hardware and master cylinder will probably run somewhere in the $1500 to $2500 range.

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