Retail Margins for Franchisees: Are franchisors squeezing too hard on the price of mandatory purchases?

Is it just my imagination or are more franchisees/dealers being squeezed by their franchisors these days on the price of "necessary" or otherwise mandated products, thus putting the squeeze on the franchisee's margins?  

Am I right in suspecting that this problem is especially severe in industries where retail margins are historically tight, e.g. the price of gasoline sold at gas stations or the price of food in quick service restaurants or c-stores?    

I am representing gas station owners who face this problem, and who are fighting back -- and for a number of reasons we would like to know how widespread the problem is.  

Is it the economy, or simply the greed of certain franchisors?