I've been messing around with Odrysian (central and western) campaigns a bit lately in sandbox, and the displayed alliance costs at the beginning of the game often don't seem to correlate to the alliances I'm actually able to make.
E.g. as Central Odrysians at the start I was able to ally with the Western Odrysians without having quite enough money spare. As Western Odrysians I was able to ally with Paeonians and Illyrians but not Paeonians and Central Odrysians, despite the Illyrian alliance actually costing more than the Odrysian one I couldn't get.
Is this because the alliance cost takes into account the income and upkeep of the faction you're trying to ally with, or is it some sort of bug?
If so, I think it'd be rather more easy to use and a bit more intuitive to have the income/upkeep etc. factor in after you've agreed the alliance, which'd make it more of a matter of being able to increase intimidation and scrape up the spare gold simultaneously than a matter of going for anyone rich instantly.
Oh, and I love the depth the alliances offer to the Odrysian starts in sandbox - really allows for some very different gameplay styles.