This achievement is cumulative, you do not have to spend $25,000 at one time.
You will get money by playing through the Adventure and the Mini-games, but a much more efficient way is to use your Zen Garden, unlocked after beating Level 5-4. The plants in the Zen Garden can be watered and fertilized for coins and diamonds. They have four stages, seedlings, small plant, medium plant, and large plant. Transitioning between stages requires you to water and fertilize the plant, and you will get an extra reward when they do so (a gold coin, two gold coins, and either one or two diamonds respectively). A seedling can immediately be watered and fertilized, and can be re-watered and fertilized 30 minutes (real-time) later). A fully matured plant can be sold, $3,000 for a Marigold and $8,000 for the rest. You can also keep matured plants and use the bug spray and phonographs to make them happy, causing them to randomly drop diamonds, but I found it easier to raise money by selling them.
Plants can be earned in numerous ways. They can be dropped as presents while playing the game. On the second play-through of the Adventure Mode, you will be given a plant (occasionally two) at least every 10 levels. Vasebreaker Endless will also give you a Moneybag, Diamond, or Present every 10 levels and I, Zombie Endless will give you one of the those three items every three levels (does not always occur, but most of the time it does). Plants can also be bought from the Shop for $2,500 (up to three a day), netting you at least a $500 profit without including the coins and diamonds awarded for raising the plant.
Anything else you used to help get money, feel free to leave a comment and I'll add it to the solution!