Your stadium is the most important part of the club income, but at the same time it could be the most expensive asset of your club. At the same time you have to treat it as investment (larger stadium capacity will generate more income), but also keep in mind that stadium needs to be kept in good condition, not to mention that from time to time you will need to pay the property taxes.
Apart from seats capacity there are other elements of the stadium that you can improve:
While investing in stadium keep in mind that the elements of the infrastructure are linked to each other, f.e. if you will improve seats comfort this will increase cost of further increase of stadium capacity (new seats will have the same comfort level, thus will be more expensive), and vice versa: increase of seats comfort is proportional to number of seats that you have. Similar story with stadium safety - it is more expensive to adapt large stadium to safety rules than small stadium.