I think promotions in AoP is way more costly than ST. Once you get to the higher ranks, promotion costs few hundreds silver!!! In contrary ST never cost that much in proportion.
At one point few real hours ago I had 3000 silver, not any more after few promotions that came after the Ice Demons + 2 Kraken encounters. For similar level in ST I could have over 1,000,000 in money and no promotions had managed to make a dent in that even when I am playing Hard.
I am playing a Challenging difficulty and my level is 22.
Star Traders Elite
Age of Pirates Elite
Star Traders 4X Elite
I love the promotion system in this game, personally- I play a Knight on challenging and it makes for a more interesting game if you decide wether to hoard your money for supplies or a ship, or to purchase that next promotion- I usually buy a promotion, then take 3-4 commissions to make up for the difference, any enemy ships get burned for silver, and by the time I make it back to where I started I usually have enough for another promotion, if not I just keep on the commissions until another promotion is offered to me. Personally I have a great connection with an amazing tailor in Barmouth, so I get great prices on my uniforms.
Keep in mind, that the more promotions you have, the more valuable your commissions become, so it evens out, slowly, but it evens out. I've found, also, that promotions get more and more spread out the more you have, so you have more time between them to save up