I don't know anything about the first few weeks/months of the show, but in the second Edd Byrnes pilot, one \"Buy a Vowel\" space was added to the wheel in the second round, and a second \"Buy a Vowel\" space was added in the third round. Players could, however, choose to buy a vowel any time they had enough money. (No one chose to buy a vowel in the pilot, nor did anyone land on a Buy A Vowel space with too little money.) And yes, the price of vowels has held steady at $250 for twenty-eight years!
BTW, in the original box game instructions, you can't even ask for vowels after there are no more consonants to guess. Everybody, in turn, gets one shot at solving the puzzle. If no one can guess, the money carries over and a new puzzle is introduced. Also, keep in mind that unlike today's elaborate puzzles, the original ones were rarely even three words long.