Here's the table I made in Excel. It shows annual cost of each level of Fort and Garrison, effect they have on raising the power of your armed forces and price per soldier.
Conclusions:
1. Upgrading Fort beyond level 1 before upgrading Garrison to the max is a huge WASTE of money.
2. Best combination seems to be Garrison Level 4 and Fort Level 1. For 18 500 pounds yearly you get a force of over 1500 Marines. Means you pay only 12 pounds per each Marine. Best bang for buck.
Compare with Garrison level 3 and Fort level 4. Same force and its cost is over 40 000 higher. Yearly!
3. Upgrading Fort to the highest levels should be avoided at all cost and as long as it's possible. It's much easier to build a fleet and retake the port from AI that stole it from you than upkeep that thing year by year.
4. The effect of cannons isn't included in this math, it's only the amount of Marines and their power increase by the port in there. Still, paying extra 46 500 per year for 40 cannons doesn't seem to be worth it.
For that cash you can put a fleet Frigates at the entrance of the port. 5x Frigates with 200 heavy cannons cost 22 500 per year + captain's salary. Construction of 5x Frigates costs 500 000, upgrading a Fort to level 4 is 720 000. Frigates can beat any approaching fleet unless it's composed of SOLs. And even SOLs they will damage so much that attack won't succeed.
Seriously, upgrading Fort beyond level 1 is a huge expense for little gain. So don't do it unless you swim in cash.
5. To check if my math is correct or to fiddle with numbers even more, you can use the Excel sheet I made:
http://www.speedyshare.com/166226517.html
Enjoy.

Conclusions:
1. Upgrading Fort beyond level 1 before upgrading Garrison to the max is a huge WASTE of money.
2. Best combination seems to be Garrison Level 4 and Fort Level 1. For 18 500 pounds yearly you get a force of over 1500 Marines. Means you pay only 12 pounds per each Marine. Best bang for buck.
Compare with Garrison level 3 and Fort level 4. Same force and its cost is over 40 000 higher. Yearly!
3. Upgrading Fort to the highest levels should be avoided at all cost and as long as it's possible. It's much easier to build a fleet and retake the port from AI that stole it from you than upkeep that thing year by year.
4. The effect of cannons isn't included in this math, it's only the amount of Marines and their power increase by the port in there. Still, paying extra 46 500 per year for 40 cannons doesn't seem to be worth it.
For that cash you can put a fleet Frigates at the entrance of the port. 5x Frigates with 200 heavy cannons cost 22 500 per year + captain's salary. Construction of 5x Frigates costs 500 000, upgrading a Fort to level 4 is 720 000. Frigates can beat any approaching fleet unless it's composed of SOLs. And even SOLs they will damage so much that attack won't succeed.
Seriously, upgrading Fort beyond level 1 is a huge expense for little gain. So don't do it unless you swim in cash.
5. To check if my math is correct or to fiddle with numbers even more, you can use the Excel sheet I made:
http://www.speedyshare.com/166226517.html
Enjoy.