Anything over 150USD is silly money for pretty much any turntable Marantz made in the 1970s. I have had a 6100, 6025 and 6170. They were pretty average, equivalent in performance terms to low to mid level Technics decks from the same period, certainly nothing near approaching the SL1100, 120 / 1200, 150 or 1500, never mind the proper hi-end Technics like the SP-10.
The only really interesting deck Marantz made before the Philips takeover was the SLT which I would pay more for because it's one of the first attempts at Linear tracking, so it has historical significance, even though it's alledged to be a turkey and was quickly withdrawn. The first real linear tracker is the Bang and Olufsen Beogram 4000, which I am still attempting to coax back into life..
These inflated prices are mostly down to sites like this http://www.classic-audio.com/marantz/mindex.html
which gets a little misty eyed in the presence of those chunky gold buttons and dials perhaps. Even http://en.wikipedia.org/wiki/Marantz
has been infected by this nostalgia it seems.
Frankly it's nonsense when you consider that Marantz under Philips ownership were making amps like this http://www.quarter-a.netfirms.com/pm-94_oak.htm
in the 1980s. Not to mention of course the excellent Philips based CD players which Marantz made like the CD94 / DAC94 , CD 12, CD7...
And then there are the Marantz pro cassette decks made in the '80s like CP430 which rather than buiding on the great skill of '70s Marantz in cassette decks, owe more I suspect to the fact that Superscope (who owned Marantz then) were marketing Sony built decks in the USA before Sony themselves set up an American subsidiary. The CP-430 is I believe really a distant relative of the Sony TCD-D5 therefore. You only have to look at this http://www.classic-audio.com/marantz/mpa.html
and realise that Marantz never even sold an open reel machine to guage the provenance of their engineering skills with tape decks.