I think the key is to never ask how much. If you don't say a word, and sit there like you do this regularly (not pointing at everything with mouth open !), they assume you know the price.
Just hand over the money and wait for the change with your hand extended. Obviously you should give a maximum of 100 pesos, no more than that. I ALWAYS have smaller coins and notes than that.
If I only have 100 pesos then I give them the money before - so have time to deal with it if any problem with the change (gua gua and publico).
In all the past years they have ever only over-charged me by 5p or 10p maximum (and that rarely happens).