Howdy Mike,
There was a change a few versions back. Our price field has to contain an integer. There was a while where you could type something else in, but it would break the calendar in areas, particularly the Filter Bar. So when we change this we were actually fixing a calendar breaking bug.
What if you tried adding an Additional Field in Pro (WP-Admin > Events > Settings > Additional Fields) with a title like “Price varies” or perhaps “Price varies based on age”. You could even list each cost if you wanted. Would that work?