For lvl 17 i'd say:

4*: around 95-100M
5*: 103-104M+

Even though i have a 4* at 103, which train very quick (almost 8 training point for 1 condition).
Click image for larger version. 

Name:	2014-03-31 13-02-33.jpg 
Views:	69 
Size:	91.7 KB 
ID:	21617
Here's a graph of the earned training points (need 100 for one skill point) for one condition point:
Click image for larger version. 

Name:	2014-03-31 13-07-38.png 
Views:	76 
Size:	57.1 KB 
ID:	21618

I simply look at the condition before training, then after training, let's say the guy went from 95 to 85 and earned 40 training points, i know that for one condition point, he earns around 4 training points.
Obviously, you need to do this many many times to have accurate ratings.

(Earned training points)/((Condition before training - condition after training))

Long process, but this way i know which ones i should keep and which ones i should sell (or try to sell... the market is a pain since 2 seasons, only 5 mins left each time you go there -_-).