There is no answer that sums it up, so here's my two-cents.
The highest tier that you can achieve is nine. You can get these by spending ten Dozer Dollars to upgrade one category from six as the highest tier to nine as the highest tier. You earn Dozer Dollars by either buying them via. iTunes or earn ingame by collecting puzzle pieces to solve puzzles. Solving puzzles awards you a set number of Dozer Dollars (I believe it was twelve per puzzle solved) and some experience (I think it was around 5000).
However, it should also be noted that after a while, the game is self-sustaining. You have more money than you know what to do with, and you start to lose interest in the game.
After you finish the puzzles, the game starts to advertise other games that the company Game Circus created, starting with Horse Frenzy. After completing the puzzle and collecting your prize (Dozer Dollars and Experience), you can scroll the page to see the next puzzle. Therefore, there is no endgame, so long as the company keeps pushing out new games. Additionally, it is possible to have six-digits worth of coins, as well as being up to level 300 (I'm not sure why I continue to play this game :P).