I thought upgrading triples the amount of cash needed to upgrade after each rank.It costs $20,475 to buy enough donkeys to make a level 5 upgraded donkey. For the upgrading it's 1 nugget or $125 for each level 1 upgrade. To get to level 5 you need 3 level 4 upgrades and either 1 nugget or $125. It took one of my characters a few hours to do it.
Edit: The other achievements after getting the level 5 Donkey are just a complete waste of time and money.