The sum you get in exchange is proportional to the army lost. With the default startup army you get very few money.
Moreover the more you apply this the more the sum you get decrease , that's not linear but the decrease is fast. I mean the money you get for the same army lost decrease. It's for example a significant decrease at third attempt, then more decrease at 5th attempt. That's the idea not the exact decrease rate.
Finally the time to run from castle to the nearest troop is far to be null. Really playing the game will earn you the money faster for real time and game time.
The point is I already tried this and give up before 10th try.
