It might also be useful to give some more details on the popularity benefits of Fan shop 10 and how that compares with having extra Fan shop staff. 

The relevant factor is not the total benefit per se, but rather the increase compared to level 9. 

From the first post, popularity increases at a higher rate than level 9, as follows:

• 50 extra per day (daily advertising)
• 260/200/25 extra per home win/draw/loss
• 205/210/225 extra per away win/draw/loss
• 20% increase in fan conversion rate

Fan shop 10 seasonal increases 

Multiplying these for a season, you get:

• 2600 extra for daily advertising
• 9215 extra for league matches (at best) 
• a little extra for cup/continental matches 
• extra fan conversions - proportional to the current popularity 

Fan conversions  

I cannot give an estimate because it varies widely with current popularity, mixture of fan types and staff levels. But here is mine as of today:

• Fanshop casual to normal fans +1653 popularity
• New fanclub memberships +2625 popularity

This is with Fan shop 9 and 2 staff @ level 8. The current popularity is 2.24 million. 

This may or may not be typical of a club at level 9, but it's probably not far away.  Except for the effect of having double staff. If I deduct the effect of one staff member then I would get:

• Fanshop casual to normal fans +1463 popularity
• New fanclub memberships +2323 popularity

With an upgrade to Fan shop 10, giving a 20% increase in fan conversions, this would give an extra daily popularity increase of +757. That's an additional seasonal increase of 39,364.

(If the club had double the popularity the mixture of fan types changes so these figures can't easily be extrapolated going forward.) 

Comparing the 39k with the above increases for playing matches it's easily seen that increased fan conversions are the biggest factor by a large margin. 

At low popularity (and fan club levels) then the daily advertising and matches are the most important element but as popularity increases fan conversions become the biggest factor in gaining popularity. Match results are still important though, just less significant. 

Comparing the effect of staff

If I added back in 1 staff (at level 10) to my level 9 Fanshop, so it was now double staffed. The extra increase in daily popularity would be +568, less than upgrading to Fan shop 10.  However, staff also decreases the natural decline rate, which in my case would reduce the loss by another 559 per day. Adding these two together gives an effective seasonal increase of +58,604. 

Thus it seems to me that the effect of the additional staff (at level 10) is more effective than upgrading the Fan shop to level 10 at current popularity levels. Although extra staff would have an even bigger effect with Fanshop 10. 

These figures are based on the current popularity of 2.24 million. Going forward the fan conversion rate would probably not increase significantly as it would reach an equilibrium at some point, but the decline rate  would continue to climb - and hence the reduction in lost fans becomes increasingly important.  The 2nd staff member is therefore even more useful when compared to the potential Fan shop upgrade. My conclusion being that double staffing Fan shop 9 is better than having Fanshop 10, but that is not to say that double staffing Fanshop 10 would be even better, or the lost opportunity of double staffing somewhere else. 

(Apologies for the ultra long post, and congrats if you read all the way to the bottom.) :) 

Fs10 with staff lvl 10 give an increase of 11% fan conversion.
600 extra for daily advertising
Fanshop casual to normal fans +2750 popularity New fanclub memberships +6195 popularity
natural decline are -9454
but after a day not selling tickets you will still gain some little popularity, without any from games popularity and even more when there is games.
don't know If fs9 gets negatives on popularity some that's especially off season.
No one will stick to doubled staff on fs, for so long.