Like most vintage cars, a lot of variables go into determining value and price. Start with what it is (Mach 1 or base Sportsroof?) What factory drivetrain and equipment did it come with (small block, Big Block, Cobra Jet, Boss 302, Boss 429, A/C, tilt-away column, etc.?) What condition is it in ( rust wise, damage wise, missing equipment wise?) What is it's provenance (solid documentation?)
From there, the assessment moves to what you want it to be when it is done? For example are you looking for: a beater with style; a nice driver; a competitive concourse trailered winner, a track car; or what?
Do not underestimate the cost of high quality rust repair, body work, and paint, especially if you are farming out that work.
Generally a project car will cost more to get it to the level you want than buying a complete car that is already turn key for your purposes.
With these things in mind, look at what done cars of that caliber are going for (not asking prices but actual sales prices). Then calculate what it will take to get your project to that level. Do the math to determine the present value for your purposes. From there contrasting value with asking price will help you narrow down the price range that works for you.
In general I have seen total turds go for $7K and up. I have seen decent drivers in the low to mid $20K's I have worked on solid cars that cost $25K and up to reach the level of very nice driver.
Go over the car carefully and report back with your findings for a tighter range of sale prices. Good Luck.
*Principal wrench on this 69 Mach I*