I don't think there is one after all.
I came into L6 with large town, most of last season I failed to fill it no matter what I charged 21,000 capacity and struggled to get 19k
This season was the same lowering prices had no effect.
I started the full seat upgrade which finished seven days ago, first game was a CL match charge $12, league $10 and Cup $10, attendance around the 22,000 which meant 3-4% bonus, with a capacity of 28,500.
Since then I have reduced prices to: League $8 CL $9 Cup £7 every game since has been 28,500 with 5% bonus.