Again, this is completely speculative and we'll all look like idiots when Disney reveals whatever is it they will reveal (which won't be a surprise to me; I always look like an idiot).
However, if a pay-per-use feature was implemented I assume it would just be part of whatever Fastpass allocation they specify be made available (be it 30% or whatever of the ride's capacity). Price is then determined by availability within that pool and time/date/crowd sizes. Very similar to surge pricing models for Uber/Lyft.
Man, it would become like Vegas with people deciding if they should spend more money on gambling or cut their losses!
Hmm, it's $3 to ride Pirates now, maybe it will go cheaper if we wait, but maybe price will be $10 later!!!