If the goal is to win as much money as possible, why would you want to “spend” any of your potential winnings on buying vowels? That’s because the goal isn’t (just) to win money it’s to solve the puzzle.
In that game, the most common letters are worth the least points. This roughly aligns with Words With Friends letter values. Depending on the source, the most common letters in the English language, in order, are ETAOINSRHL or EARIOTNSLC. Those six letters are among the most common, but they’re not necessarily the most common nor are they in the right order. And given that the “free” letters in the bonus round are RSTLNE, you’d assume these are the most common. Wheel of Fortune contestants typically gravitate toward consonants they think are most likely. This may sound obvious, but it’s not as straightforward as you might think.