Throwing money at a mission doesn't guarantee success, as witnessed by the failure of the $1 billion Mars Observer mission in the early 1990s. For that matter, the Hubble was a $1 billion + (a lot) blunder until they managed to launch a Shuttle repair mission to work around the screwed up optics. That repair mission probably cost at least $500 million.

Nor do all good missions require a lot of money, as shown by Clementine, MGS, and some others. As in all things, a healthy balance between budget and mission objectives is needed. Mars Polar Lander tried to do too much for too little money. There have been many Discover class missions in the space program and many of them have worked quite well.