Don-x wrote :
Its a simple maths and its not a facility you can depend on for better income per season, you can build it after and expect it to cover some expenses like staff/facilities.
With 50k stadium and max level fanshop @$20, if average 70% fans buy things.
50k all home matches equals 700k×0.70×$20 =$9,800,000.
Training center + youth academy is more profitable in the long run.
I have 15k stadium when my youth academy hit level 12, i also have the best youth players from the academy no regret to opt against building fanshop till date.
One correction, you have 15 home matches in the league. So in the calculation you did above it is actually
15 * 50k * 70% * $20 = $10.5 million per season minimum on average.
There will be more matches in the international competitions if you are a top team participating in those. National trophy and league cup doesnt count towards this as they are played at neutral grounds.
But you are right, you should upgrade fan shop only when your stadium has very high capacity. In the beginning it is wiser to invest in youth academy and training centre (and may be even media office, haven't done that math yet)