Use the TL of the currency that the polity uses. 3I uses TL-15 to make their currency. All polities use the highest TL available to them to make their currency. It makes it harder to counterfeit. Give it a few years and the 3I's currency will be TL-16.
Sure, minting the Third Imperium's currency is at TL-15, but that is not important. What is important is the strength of the economy which supports the currency -- and much of the Third Imperium is nowhere near TL 15. TL 12 is the standard for most Starports and general usage ships, for one example, and that would not be true if the entire economy was TL-15. I'll look around a bit for the quote, because I do not remember exactly where I saw it -- but there was a statement that the 3I was, on average, TL 12. The Aslan and the Floriani were mentioned as well (and maybe the Zhodani as well); none of them are 'averaging' at their maximum value, so the currency should not be assumed to be based on their maximum TL.
Aslan are even wonkier. There is no Aslan currency. Each clan uses their own and their relative values to other Aslan currencies are a function of diplomacy instead of economics.
I am sure there is Aslan currency; the females are very keen on commerce. As you say; it is probably clan-issued scrip rather than Hierate-wide -- but some of the clans have extremely extensive holdings. One aspect that might be fun is to use mostly the clan-currencies of the lead clans of the nine major power-blocs in the Tlaukhu -- each having a slightly different (and fluctuating) value. Clans in the Trojan Reach might tend to use clan-scrip from the eight clans that form the Ya'soisthea instead.
It is about Manufacturing Plants and not Trade. You are correct in that. One of the reasons I do not want to change the value of the currency (especially within the Imperium) is that if, you then apply it to trade between worlds, the Trade System in the WBH no longer works. If you have a WTN 3 world at TL-10 and a WTN 3 world at TL-15. The BTN in the WBH would say that the BTN should be roughly 7 or 8. If We add the currency exchange to the WTN, things get wonky.
So don't bother applying the exchange rate to the BTNs at all; I think that might be entirely justified in terms of 'BTN is about the trade of goods'. GURPS Far Trader didn't, as I recall, apply the exchange rate to BTNs either.
[Edit: Back at TL 3 Terrans figured out how to make cast iron; our 10 dTon TL 3 'Smelters' took two dTons of Common Ore (at 1000 TL 3 Cr per dTon), one (TL 3) worker, and 5 power, and transformed it into one dTon of 'Common Raw Materials' (at 5000 TL 3 Cr per dTon) each day. Cast iron is a TL 3 product, and we do not make as much of today at TL 7. If we did, we would feed a 10 dTon TL 7 Smelter two dTons of TL 7 Common Ore (at 1000 TL 7 Cr per dTon), with one (TL 7) worker, and 5 power -- and we would still get 5000 Cr worth of 'Common Raw Materials'. But at TL 7 those 'Common Raw Materials' are high-performance Steel alloys; if we were to settle for just TL 3 'Cast Iron', we would get 5000 TL 7 Cr worth of TL 3 goods -- 8.21 times as much (at an exchange rate based on 25% improvement in productivity per TL).
So there is no need to describe and enumerate all the little details which contribute to TL 7 having better mining, better temperature control; better assaying for purity; better understanding of metalurgy. The Smelter is simply X dTons, takes Y workers, Z power, and two dTons of ore, and makes 1 dTon of 'Common Raw Materials'
of the same TL. All the TL advancement is handled, automatically, in the background, by the exchange rate. /Edit]
