I bought my MM about 4 months ago, the seller actually had 2 of them. He only had one listed on CL for I think $500 and someone else went to get it before I had a chance to. The other person saw he had a pretty minty looking one (not for sale) which he convinced the seller to let it go for $900.
I wonder who got the minty one for $900 from him? probably a KLOVer, they also purchased several System I kits (I think 5 total and NOS complete kits) including MM, Indy TOD and Peter Pack Rat for I think around $1,300 more - who was it? I wanted one of those PPR kits, Dammit!!
I wound up getting mine from him (the original listed one) for $400 (sides are painted blue and has Road Blasters short Gas Pedal), fully working with clean CP and Track Balls. He said he found a spare board which he gave me for free, but wasn't sure if it was MM or not. When I plugged it in to check, it was an Indy Jones TOD! So I wound up getting a nice working matching IJ TOD CP to match for another $50. Now I'm looking to get another Atari System I, probably Road Blasters (the least expensive) to have more options.
So in summary I would say fully working probably $500 to $900 depending on condition, then you could negotiate the price depending on the seller. Not working or needs TLC maybe $300-$400?