Currency rank 1 doubles the "do nothing" gold income that you get (it's a base of 4, currency 1 adds 4 more).
This is such a massive boost that someone must immediately get currency as soon as you reach War Rank 3. It pays for itself for the person who bought it in 7.5 minutes. To everyone else it's free gold and massively adds up through the rest of the match.
Currency Rank 2 and 3 on the other hand are utterly worthless. Rank 2 only increases your income by 25% (and Rank 3 only 20%) and are quite expensive. Rank 2 takes 25 minutes to recoup the cost for a single person. Rank 3 takes 35 more minutes. Not only is that about the length of most games but you have to wait for war score 5 and 7, which is way too late anyways. Even dividing the cost among the team (on average 3 people) it's still worthless - you're not gaining enough.
Can we think about these upgrades and work on making them useful? If Rank 2 and 3 added an *additional* 6 and 8 gold to the income then they'd certainly be worthwhile. (Then a single person would recoup costs in 8.3 minutes and 8.75 minutes respectively, which is way more in line with currency 1! And the team would obviously greatly benefit)
This is actually a pattern with most of the citadel upgrades, rank 1 is basically the only good economical thing to buy, aside from creep damage/health upgrades which are only good late game because of how many creeps they are affecting.
Fortified structure - Rank 1 = 10 hp/sec regen for 600 gold. Rank 2 = 15 hp/sec regen for 1800 gold. This is both diminishing returns AND more expensive? Why?
Building firepower - Rank 1 = +10% damage + splash for 500 gold. Barely anyone gets this because towers ALREADY have splash (something that always confused me, are they supposed to or not) Rank 2 = +15 damage and larger splash for 1500 gold. Diminishing again for larger cost. The damage needs to increase exponentially if you want towers to actually be useful at defending.
Experience, Death Penalty - Both increase linearly but are very expensive. The experience bonus should be much larger as at the point you can buy them 10% boosts aren't really going to do anything at the higher levels.
Armory and Blacksmith - While these get very expensive they are actually worth it once you have full creeps. However I think that static increases of damage/hp to the creeps would be more effective than % gains, as the upgrades could then actually help the minotaurs and archers early game, since they are so worthless now. As it is no one gets these upgrades until they have giants - and why would they?
Trebuchets - The value of this upgrade greatly depends on your map, and it should not. On Cataract there is exactly ONE fortress. Getting trebuchets for it is not worth it, at all.
Finger of God - Decently good, I think this is fine.
Creeps - Something has to be done about the repercussions of getting creeps. Getting them as soon as you can sounds in theory like a good thing - more per wave, you'll push them back. But if you do this you end up feeding the enemy more exp and gold (especially from priests). Angels are very lackluster, but priests catapults and especially giants are quite good. Most games I see people waiting until they can get priests angels and catapults (if not giants also) in one large burst all at once. This seems most effective to me also.
As you can see most of the citadel upgrades are pretty bad but a few are absolutely crucial. This should really be looked at by the devs. The citadel upgrades are one of the key features that sets Demigod apart from Dota - the ability to spend your money to help the team. It should really be fleshed out and balanced properly.