The thing you need to keep in mind is that each building level gives you 10% improvement or so. So lets do the math.
A basic production province can give you 3,000 tons of stuff per day.
A level one industrial complex adds 10% = 3300 tons
Level 2 IC 20% (adds 10% to prior) +300 = 3600
Level 3 IC 30% +300 = 3900
Level 4 IC 40% +300 = 4200
Level 5 IC 50% +300 = 4500 tons per day.
Then you build Infrastructure:
Level 1 Infra adds 10% So +300 = 4800
Level 2 skips to 30%, so +600 = 5400
Level 3 gives you 50%, another +600 = 6,000 tons of product per day.
The only thing that detracts from the 6,000 tons possible is Morale. It does not seem to be directly related as a province with say 92% morale seems to give 98% production, so there is a formula that I haven't taken time to analyze.
Now if you have a double resource province that is the one you want to upgrade first, as it will give you twice as much return for the same investment.
The other way folks go wrong is upgrading non-core (captured) provinces before the core provinces. Non core provinces will only ever produce 25% of your original core provinces. So instead of a maximum of 6,000 tons per day you have a maximum of 1,500 tons per day. Those are worth investing in when you have a shortage of something, but ONLY when your core provinces are also being invested in.
So once you have all the upgrades you are getting twice as much production per province per day. NEVER stop building until your economy is maxed. A big economy can support a big army. A small economy can only support a small army. To quote one of my least favorite American Presidents: "It's the economy, Stupid!"