I get what you're saying, but my point would be that the rounder the number you use, the more you're anchoring your negotiation to the rounding point, and the m

I get what you're saying, but my point would be that the rounder the number you use, the more you're anchoring your negotiation to the rounding point, and the metric being used here seems to be the outcome of the negotiation. If someone says their price is £45,765 then the anchor is to the single pound. If someone says their price is £45,000 then the anchor is to £1,000s. I think there are both things - complexity and anchoring - at work, but I'd say the anchoring to the rounding point is actually most salient here and don't think it's necessarily much to do with perception of complexity, as attractive as that might be as an explantion.