First, there are only one type of gas station audit that I do for under $20. It is Circle K and appears only on Prestomap. It is unrevealed with maybe 3-6 photos. Quick and easy.
P66 et al These start at $22 in my area. Easier than others. One pump pic makes it quicker. $10 reimbursement to be used as I see fit.
Shell: $14 with two gallons of gas. Currently bonused at $20. Requires photo of each pump, which in a busy station, can be difficult. Requires photo of two restrooms if they are present.
Exxon/Mobile: $12 with $3 reimbursement ($2 gas). Picture of each pump, no overall canopy photos. Rarely do one of these.
BP/Amoco: photo of every pump. Low reimbursement. Rarely do one of these. Fee starts at $18.
Marathon/Arco: $10 and $2 reimbursement in store. One of the Easier ones. Usually I do when bonused to $15. Used to be $7 flat fee. Photos of one pump close up and far away.
Valero: $12 and $5 reimbursement. I do when bonused. Only 4 Valero stations in my area.
Generally, I do P66, Marathon and Valero stations on a quarterly basis. Circle K is monthly. $5 plus $6 reimbursement. ($3 gas and $3 in store. P66 is my favorite, but losing stations due to rebranding in my area. Circle K second due to ease. Marathon 3rd when bonused.
Shell would be 4th and BP 5th. ExxonMobil last.
Do not read so much, look about you and think of what you see there.
Richard Feynman-- letter to Ashok Arora, 4 January 1967, published in Perfectly Reasonable Deviations from the Beaten Track (2005) p. 230