I would go up to $1,000, any more I would be looking at new. In 2008 I bought my SQ172 new for $1,430. Mine does not have chains. Have never had a problem with mine. The SQ172 is listed on TractorHouse from $995 to $1,950. There are SQ160s listed from $1,250 to $1,900. Before I paid a high price used I would get a new one with the 1 year limited warranty and the 5 year gearbox warranty.
They do seem to hold their value, for a reason. For comparison a new County Line 5' cutter from TSC is $1,400 and a 6' is $1,700. The SQ160 has a 11 gauge deck and the County Line is thinner at 12 gauge. The SQ160 has a 65 HP gearbox, the County Line has a 45 HP gearbox. The SQ160 weighs 100 pounds more at 636 pounds.