I have this weird feeling that Silver Serpents and Gold Krakens are actually independent currencies from different parts of the Blue Flame Isles, but that might just me being weird.
I'd have said that based on the rate of needing Serpents and Krakens for Gangplank and Black Market Brawls that a conversion rate is somewhere around 1,000 Serpents = 1 Kraken, but I dunno.
On the currency and Runeterran economics thing, I'd imagine that (almost) every nation has their own independent currency.
Noxus has Securi and Gold Securi.
Bilgewater has Silver Serpents and Gold Krakens, presumably they also have Bronze somethings. I don't know any sea monsters that start with the letter B though.
Old Shurima has an ancient form of currency but likely now functions off of Securi and whatever Demacia's currency is, depending on how close those nations area to any given part of Shurima. The old currency is likely defunct but considering it's historical worth might be worth more now than it was in the empire's heyday.
I imagine Freljord doesn't have currency, and is focused more around trading. Ashe might make changes to that though with her politics and stuff.
Piltover and Zaun might have the same currency, considering they're quite close to one another, but that also depends on what's between them, be it a massive bay, swampland or whatever.
Considering we don't actually know too much about Yordle culture, it's debatable whether Yordles and their capital of Bandle City even has a currency at all.
Ionia likely also has it's own form of currency, but given their spiritual culture it might not be a thing that carries weight in some parts of the nation. I'd imagine rural Ionia has communities, temples or what have you that don't function with any currency.
(Influence Points were "currency" of the Summoners in the old lore)