Hestecrefter... Good job on buying the chicken at 140 pesos instead of the 250 pesos, and you have made my point by knowing what the prices are or what you expect to pay for meat, vegetables and fruits, an expat will never learn if he keeps sending someone to buy his things. Example, when we were visiting as tourists the in-laws would get a 50 kilo sack of rice (1993 price) for 3,000 pesos that's what we had to pay and then late on we found out it more like 1000 pesos, another time a helper would run and get my local made hamburgers the price turns out to be 10 pesos but he was telling 15 pesos and he still got a tip for running, not big money but it adds up.
If the price is too much I won't buy it, when you calculate the exchange rate and what you get doesn't' make sense or you'd be better off buying a whole rotessiere chicken for less than an uncooked one you have some issue with the vender and you begin to understand the value of things and what you are willing to pay for them, I think the only time I might pay more for an item is when it's not normally sold in any store and I'll never see it again, things like quality computer chairs at the Japanese surplus, real steel parts for bikes... today I bought a bracket for my bike basket, my bike is surplus from Japan and I had everything for the basket except the upper basket bracket, I paid 100 pesos for original quality.
How is it possible for an expat to understand prices if he doesn't live and learn directly in the markets or in the grocery stores, impossible unless we are constantly buying items, and I don't buy vegetables or fruits in the grocery stores the prices are doubled but from my neighbors mainly or the occasional fruit seller but some items such as lettuce or cheese I have to buy at these chained grocery stores and it's expensive.
Another note is if you watch how Philippine citizen or (wife) will ask for prices and the reaction they keep it respectful and friendly even if it's high and then work with the price see if you can get if for less, most times you can it comes down to many factors.