I, personally, am not opposed to upping the vowel "charge," and frankly, I thought it was assuredly gonna happen about 5 or 6 years ago, when the bonus wheel came in, and during the very short time 10k was on the wheel in round 4.
However, $250 for a vowel is an iconic part of the show, no doubt, and if it's changed, people will definitely notice. Many may even expect if the cost to buy a vowel will increase, the values on the wheel will also increase, the center of our discussion.
I have a simple solution, that even Joe and Jane Sixpack can keep up with. For rounds 1 and 2, keep the vowels at $250, then in round 3, have Pat give short explanation that since the values on the wheel have been increased substantially since round 1, vowels are now increased to $500. Have him note that there are 2 $1k spaces on the Wheel in Round 3 (mystery wedges). Then, to appease those who'd want some more compensation on the wheel itself, throw a single $1500, $2k, or $2.5k space on the wheel in round 4, and say done.
That should be enough to make everyone happy. As for ppl who would complain, in the most drastic monetary change, where in round 4 (which 4/5 times turns into the speed round after about 2 or 3 spins anyway) a $300 was replaced with a $1500 (as opposed to replacing an 800 or 900, which would be a lot less drastic), the wheel's average value in Round 4 would go from 740.48 to 797.62, a mere $50 more, that will 4/5 times be of consequence for less than a full round...