These prices seem a bit too steep in between upgrades.
First upgrade: 15k
Second upgrade: 50k
Third upgrade: 120k

More steady increments [in comparison to these seemingly exponential ones] would make more sense for something like this since it's something that is continually stacking[?]. Perhaps an additional 10k [or 15k] with each upgrade would be more reasonable.

Although, this may tie in with earnings from Umbra Forest when it releases, if money rewards are higher in there.

@Draco

Maxing out your Stables or Inventories was not intended to occur with just Viridian Meadows unlocked; therefore the prices seem very high at this point in time.

As levels increase, there will be more ways to make money and you will make more money from the same methods. Items will sell for more, you will be able to earn more from Jinxie's Jamboree (or other games), missions will reward more gold, treasure boxes will reward more gold, and mobs will drop more gold.

So while it may seem like a lot of money at this stage of development, the prices are actually set for future stages of development.