Buying most upgrades gives the chance that they will appear as you run. What is unclear to me is whether I can influence the likelihood that I'll get a given powerup by NOT buying the others. The other possibility is that not buying a powerup just means there is a blank spot where I would have gotten that powerup.
Yeah, just corroborating what Usernumbererror said; I'd maxed out Coin Magnet, and I accidentally pushed Mega Coin to level 1 when I'd meant to leave Magnet as the only upgrade. Now I get Magnet exactly half the time and Mega Coin the other half. Same number of Power Ups per distance too, roughly. I think Power Ups have a preset probability of showing up on each segment of the run (same way obstacles and turns have a probability of showing up each segment), and when a Power Up is rolled positive in a certain strip, the game runs an even probability function as to which Power up will show.