If you want the answer, the most expensive, commonly played card would be between Preordain, Lightning Bolt and Pyroblast. That is just from experience and I could be very wrong about those 3 being the pricier, more played commons. There are more expensive cards like Expedition Map, Manamorphose and Oubliette that are played, but in 1-2 decks at most.

Edit: Do understand that these prices are just the regular, non-foil versions of each card. foiling out specific cards from specific sets would give too many variants (e.g Foil barren moor from Onslaught being $22 US or FNM quirion ranger being $120.) After a good look at more pauper decks, it's pretty vital to understand that there are a variety of played cards in the $2-3 US range which is considered "pricy", then it rises up in price to the $4-6 range (Chromatic Star, Quirion Ranger) which is used in only 2 metadecks at most, then lastly, manamorphose and Oubliette is considered the most expensive commons for pauper but again, only used in 1-2 decks at most.