[Endoperez]

I usually recruit a more expensive mage in the beginning, as the early fast research can potentially help a lot. Generally I find cheaper mages better, though.

With Jotunheim-Midgård Seithkona (90 gp, Astral/Death/Nature), I couple their research power with Magic scale and pretender able to cast some devastating Evocation. They research e.g. Falling Frost... and continue up to Nether Darts, making themselves a major military force!
Also, while it is expensive, Initiates can be very effective when summoning lots of Will'O'Wisps. And Atlantis' W1 mages become the death of anything alive when you reach the dreaded Quickness-Frozen Heart combo. With Man, both Bards and Daughters have their uses: Bards as spies and Daughters as Vine Men/Ogre summoners. I think it's not worth it to buy the cheap mages just for research. You've to come up with a use for them. They don't have to be as cost-effective at it, though, as their main use should be research.