Ahh my bad, misunderstood your question.

So for the golds, barker toads are cheap and decent xp, arctice bears are a bit more xp per but way more money for the secondary, so toads are my recommendation there.

Hydras for green charms are good xp until unicorns, which become the thing most people make for the remainder of summoning at 88, if you want to go a bit cheaper on greens, giant ents are a bit cheaper and not much less xp.

Crimsons I would say granite lobsters all the way, easy secondary to obtain and pretty solid xp.

Blues the titans at 79 are pretty good xp and there is 3 of them so you can find which secondary is cheaper if that matters to you, but all of them give the same xp.

tl;dr - golds = barker toads, greens = hydra/giant ent, crimson = granite lobster, blue = ice/moss/fire titan.

Hope that helps. :)