It seems to fluctuate quite a bit.
It seems to fluctuate quite a bit. Going to Cozumel last December it was $163 and coming back it was $115. It's $10.50 US if you pay with US dollars but only $8.95 ($163 MX) US if you pay with pesos. That's why I always leave Mexico with a few pesos. I'm quoting the current price on the Ultramar web site but like you said that is not the actual price.
Yes, I was expecting $163 and was pleasantly surprised when it was less. And, I do the same with pesos for the same reason!!