You should look at the "Which ally should I upgrade?" post to get an idea of how much it costs. For example, you'll see the spear horseman with the gem costs to the right. The game follows a pattern and through deduction I can determine that the upgrade to level 10 costs 36 gems because the level 11 cost appears to always be double that of the level 10 cost. Levels 5 and below only cost gold. Therefore a spear horseman costs 16 + 22 + 28 + 32 + 36 + 72 = 206 gems to get to level 11. I just haven't updated my chart since no one has verified it for me and I didn't keep records of it before I upgraded mine to level 11.
Comments
Spear Horseman (2.5, 5)
5/850/240
6/1025/315 16
7/1230/440 22
8/1475/615 28
9/1620/850 32
10/1800/1285
11/2070/1940 72
How much for level 10 horse spearman?