A degree would absolutely, positively, beyond any shadow of a doubt improve my chances of increasing my income. At my current job only about 7%.
But the tens of thousands of dollars of debt I'd have to take on to get that degree at 4.5-7% interest (or higher)... yeah thanks but no thanks.
So, anecdotally at least, I can agree that there are pretty diminishing returns to getting a degree. While I'd be all but guaranteed to increase my income, I'd be paying out more though:
a 20k 10-year loan at 6.8% (finaid.org suggested this %) interest means I'm paying 27,619.31
a 40k 10-year loan at 6.8% interest means I'm paying 55,238.63
At my current income the 20k loan, bumping up one position and considering our raises inside a position are limited to less than 3.5% annually based on a rather arbitrary 'merit review'. It would be 7-10 years of my increased pay just to cover the loan, assuming no taxes were taken on the increase but also factoring in insurance increases. That means in 14 years I could start keeping more money from that 4 year degree if I could manage a 20k$ degree.
Ha!