This is a table of the gold required to train stats.

You increase a stat 5 points at a time. The max points you can put into a stat is 150.

The cost obeys the following recursive formula:
gold_needed(y) = gold_needed(y-5) + 10 * (y-10) + 25 where y = 5x for y >= 15,
gold_needed(5) = 0,
gold_needed(10) = 25

You get the point.