Figure out how much the same model refurbished would cost you from apple's refurbished store or via best buy clearance or similar after sales tax. Subtract 15% from that amount. Consider that to be a strict upper bound on price. Subtract more if you prefer, but around the 10% or less mark, I would just buy from a retailer. Also note that "like new" is different from new. It can look clean yet have keyboard wear and battery aging which may not be visible. If I use a laptop lightly for a couple years, it may look new, yet its battery is not new, even if it holds a full charge.
I just noticed you said base or updated it to mention that. I'm still not sure if base means the older style or touchbar (guessing older by its price). Either way, I assume that your intention is to hold it until the new ones come out, and you would like to minimize the loss in value over this time period, assuming a lack of unforeseen events. In that case, I would stick with the formula above.