It should be cheap enough to afford for any one unit to get to the next tier.
I would think, if you are doing the cost the way I think you are, that 100 gold per tier would be reasonable. Maybe a little more in the endgame.
Upgrading my whole army should be something I am constantly doing when I have surplus Gildar.
Right now it seems to be getting skipped when you balance things as it is somewhat of a tertiary issue. I nonetheless think it should be addressed.