Anatomy of Industry
Every 1% of GDP put in = .04 points.
100 population * 5 Industry = 500 GDP. 100 income.
All into Industry is .20 at first, but then becomes .8.
An easy way is (money spent / turn start GDP) * 4
The Industrial bonus adds +20%, so an Indy nation would get more to that value, for .96 Industry at the cost of 100 gold. So they'd become 596 GDP, increasing their income to 119. Even more if they raise taxes.
Industry grows at a rate of 10% per turn for normal majors and 12% for industrial majors, so it begins to rack up interest quick.
Since population grows by 5% for majors, there's further interest. By next turn, that nation would have 105 Population and 6.67 Industry, or 700 GDP - 140 income.
Industry usually pays for itself within 5 turns or so. It is even more valuable if you have clients - your industry density determines how much industry you can absorb from NPCs, which often are industry rich.