Two similar networks are rarely the same job
Two businesses with a similar headcount can have completely different networks: different cabling, different ages of active equipment, different leftovers from past changes. A price list can't capture that difference, because it's written before anyone has looked.
Anyone who still quotes a fixed number is either pricing for the harder of the two cases — so the simpler one overpays — or pricing for the simpler case, which turns the harder one into a loss the provider recoups through add-ons.