The Treasury, Moneylender and Bank buildings have a rather interesting property of causing exponential growth. Say you have 1000 gold in savings and no income except for the 3 buildings above, which will generate 7% = 70 gold of income. Not that great, right? Well, wait 102 turns, don't spend any money and you'll have one million gold, with an income of 70000. Wait another 102 turns and it'll be 1 billion gold in savings plus 70 million income per turn..

Even if you only build the two cheaper buildings for 4% interest, that will leave you with 7 times as much gold as you started with after 50 turns. I think this could make for some interesting building strategies. It might also be why some players thought the AI was cheating, buying lots of stuff even though it only had its demesne left.

Max income from these buildings capped in cfg file, 500 by default.
So, sometimes there is no point in upgrading your 2% building at all.

On I side note, I suggest to enable in cfg lake shipyards - AI have this enabled by default. (Shipyards near 1-tile water).
More precisely - AI haven't had check, which prevent him from building shipyard, unlike player. After some whine author added this option.