Where did I claim everything costing the same contributes to versimilitude? It's used as a baseline and reference so that I'm not just pulling numbers from my imagination when something is asked for... prices can fluctuate based on many factors but, IMO, the amount they fluctuate by and the reason is what sustains versimilitude in this area, having a baseline helps me more easily improv this. It would break my versimilitude if my character enters a shop one day and mules cost 3 gp's and after adventuring for a couple of days, he returns to find them now costing 20gp (especially if the DM has no reason and really did just pull the numbers out of thin air and had forgotten since last game). Now as a DM if I have a base cost for these things I will only ad-hoc the price a little above or a little under which can reasonably be accepted by most players without breaking their vermisilitude...even if there isn't a reason that mules went up by a coin or two except the shopkeeper wants to make a little extra.
I mean I guess as an argument I could claim that having wildly different prices for the same thing every time it's bought or asked about doesn't contribute to versimilitude either... and it would be assuming there is only one way to use your imagination for creating prices out of thin air which in the far end of the spectrum as the one you made above.
Technically you didn't claim anything of the sort. But that's what the D&D rulebooks give you, in general. So I don't think it's reasonable to assume that a book of mundane items would give you a price list with set figures, since that's what previous ones have.
As for the mules, the one you got for 3gp was a broken down old one that the seller didn't really think he could get money for, so he was delighted to get rid of it to someone. Since then, he's been told that the local baron is preparing a military campaign and will need a lot of mules, so he's reluctant to sell any to an itinerant adventurer without gouging him for as much as he can. Simple enough. Not that I'd normally expect do it that way, since I'd always try to keep a price record around, but a difference like that is less than I've used in some circumstances.