I don't think that somebody would give you how much extra you get in numbers, if that's what you are asking, but every single manager that upgraded his stadium will tell you tye same, they didn't regret: they noticed its influence and their finances improves with them.
The best quality stadium is a completely different story:
It's far more expensive (5M) and it increases your expenditures, so every single seat you don't use is an extra burden.
I think the best stadium is only worth if your stadium isn't bigger than min attendance rates or if all your league opponents have really high rating and you are sure that you will play at stadium filled always.
But the jump to very good doesn't have cons, it's an interesting expenditure (1.5M or 2M, i don't remember and it's hard to check on smartphone) but it only grants you good things, without anything wrong.