Although this wiki page is old [see link below]
Stadium - Top Eleven Official Wiki
It has images for Top Eleven from the old version, but I'm assuming that the details still hold true.
This wiki page is missing the upgrade details and costs for each upgrade.. and the higher you go the more you've got to splash out!

I've currently got a Level 12 stadium with a 133,000 gate and I get around 65-75,000 [half full] at present. Not upgrading for a good while yet either. I had to do this to get the full upgrades on Medical and Training "rewards" which is what I was after [impatient or what!] Everything else is now maxed out, apart from doing any further upgrades to the ground which will take nearly a season for each upgrade, if I've got it correct, plus a shed load of money. I'm keeping this for wages/transfers etc. Would love to stop getting those annoying reminders onscreen telling me to upgrade.
Upgrading the current seating would take between 7-17 days and cost between £65-154m whereas upgrading the stadium would cost me 199m and get me from 133K up to 175K (way way more support than I'd get for a long long time) but I'd love to know what happens if/when you complete everything in-game. Do you still get them or not? lol

My stadium
Click image for larger version. 

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