Acumen is a very interesting concept.

I also only give titles of provinces to govs with 4+ acumen.

Also something that you might not have noticed.... Some of the other titles also give an acumen increase, ie Archbishop of westminster, so by giving that title to a General as well as the province title gives him a double bonus. His acumen can jump from his initial 4 to 6. Hence a lot of money.....

Add to this the trading buildings, shipping etc and the province really makes loads of money....