Coincidentally with my Troop management tip, post-1.0.7 I’ve leveled every kingdom to 2.
Say you have 19 kingdoms, one of which is above level 1, and you’re aiming to get that one to 10 first. It only costs 9,000 gold to level 18 kingdoms to level 2 which doubles your tribute income. That’s not much of a dent in the 122,500 gold you need to get to level 10.
Of course, the more often you get the hourly tribute check, the more sense this makes.
I wonder where the break-even point is. 18,000 to get all kingdoms to level 3 doesn’t sound that expensive considering how many tributes you’ll collect on the way toward leveling several kingdoms to 10.
If all you care about from tributes is gold, then just apply the above thinking to the good gold-producers:
Kingdom Name  Gem Mastery  Stat Bonus at L10  Tribute Rewards
                                                  Gold  Glory  Souls
    Khaziel            Brown         Life          250     0      0
    Broken Spire       Brown         Life          200     0      4
    Stormheim           Blue         Life          200     2      0
    Adana             Yellow        Armor          175     2      2
    Maugrim Woods      Green         Life          150     2      4
    Zaejin             Green         Life          150     3      2
    Sword's Edge        Blue         Armor         125     5      0
    Mist of Scales       Red         Life          125     0     10
    Pridelands           Red        Attack         100     4      6
    Forest of Thorns   Green        Attack          75     1     12
    Divinion Fields   Yellow         Life           50     6      4
    Ghulvania         Purple         Life           50     0     16
    Karakoth          Purple         Magic          50     2     12
    Wild Plains          Red        Attack          50     8      0
    Pan's Vale        Yellow         Life           25     8      2
    Grosh-Nak            Red        Attack           0     2     16
    Khetar            Purple         Armor           0     0     20
    Whitehelm         Yellow         Armor           0    10      0
    Zhul'kari          Green        Attack           0     5     10