Moto Guzzi's in BC are fairly expensive compared to just across the border in WA. Just about any full-size Guzzi in working condition will fetch something in the $3500 range. The sidecar is a blasting cap in the whole valuation process, as it has a value of it's own, plus the value of the worked-out combo (if it is worked out). Some might reduce the value for the workload the hack has imposed on the tug.
Some say the Quota is the best big-block Guzzi ever; but it should not be confused with the BMW R1150 and 1200 GS models, which are worlds 'newer' and much lighter, more powerful, and capable. As long as that comparison is avoided, then the Quota is fine. Assign the Quota a value of $3500 and then add to the figure any evidence of careful maintenance and love and care. Then add the value of the sidecar, always at least $1000 it seems, and add to the figure any evidence of careful set-up and love and care. Subtract value for ridiculously high mileage, and you should be in the ballpark. $6000 is just above what I would imagine the combo would be worth in BC; the last Quota I saw for sale here (I think) was asking $4500, and that looked a very solid bike. If the asking price was $5500, I would think the money was fine, and that only the desire remained in question. $6000 certainly isn't outrageous, and could even represent excellent value, if the bike was a star.