Conversion of my Pesos to real spending money?

How do you compute the conversion rate?

Pesos to Dollars?

How do I compute it?  Is $61pesos a lot for 980ml of half and half?  What is 235pesos?  If 700ml of Captain Morgan costs 174pesos, how much is that?  Here’s MY method, right or wrong.  I take the 174 and round up - 180pesos.  Next DROP the last digit - $18.  Now cut it in half - $9US. It's always a little more than my estimate, but I'm close.  It’s ‘close’ but my phone calculator converter is exact.  But when trying to figure out how much something costs I do it in my head like that.  The Captain Morgan is $10 with todays fluctuating market rate.

It gets tricky when trying to figure a tip!  What am I a Mathematician?