[(85-70)*120,000+80,000(85+13+16.74)]*20-52*(1,208766-618,701)=188,900,620 increase in yearly income
Assuming you play 1-2 cup home games per year (with 2/3 income per home game), you'll earn money from increasing your ticket prices in addition to money from selling more tickets (including merchandise and food and drinks). You'll also pay a bit more in maintenance fees for the stadium. It might then take about 7-8 seasons for it to pay for itself, though it will help out a lot in the long run. As for the interest, you'll earn back at least half a billion while waiting for it to be built.
Keep in mind you'll need to be able to sell 200,000 seats per game. You can check this with the calculator included with the stadium. If you're not there yet, you'll have until the stadium is built to get there, and you could always double the staff in the fanshop to expedite the process if necessary.
Also, don't underestimate your catering.
At level 10, 3.2*200,000*20-(241,140-130,346)*52=7,038,712 additional per year
Adding one 10* staff member at level 10 catering and level 11 stadium: 19.94*(1.1*1)*200,000*19-37,253*52=81,412,044 per year
Adding a second 10* staff members at level 10 catering and level 11 stadium:
19.94*(1.1*(1.1+1))*200,000*19-37,253*52=173,096,164 per year on top of the first staff member