[Saber Cherry]

Once you include an Owl Quill (+3) and the +1 exp bonus you get after 10 turns or 2 battles, sages are not so advantageous.

Sage, 1 star, owl quill: 11 rp @ 90g
Generic 1-magic mage, 1 star, owl quill: 7 rp @ 60g

That comes to .55 rp/gold/turn versus .57 rp/gold/turn, which is trivial. I always give my researchers quills unless I have no access to air. Really, one of the big plusses of sages is that you get a random pick for only 90 gold, which is great for opening up new possibilities.

Edit: I can see sages going up to 100g or even 110g, just because they are such a nice combo of a research bonus and a cheap random pick. However, I think the game plays out fine without changing them at all. If I had a choice between a sage province versus a druid province versus and amazon province versus a bloodhenge druid province... it would probably be a toss-up.