I bet you can somewhat quantify this with an experiment. I mean, Dave at FB was willing to ride RotR 30 times to get the line-science down, so someone like him could maybe do this.
The experiment would take 14 trips. Go on each of the 7 Tier days WITHOUT purchasing a LLMP, then go on each of the 7 Tier days WITH purchasing a LLMP. In order for the experiment to be somewhat controlled, all days would be between certain set hours, like noon-6pm.
Then count how many attractions you can ride.....go at it however you think you'd maximize the number as any vacationer would. Then for each of the 14 days, take the dollar amount you spent (base price or base with LL) and divide it by the number of attractions you rode. That would give you a dollar-amount-per-attraction value. Then find the lowest value and that would be the sweet spot. Tier-4 without LL for example. That's the day you wanna go to save the most.
There's hundreds of other variables to factor in that might skew the results (time of year, events, weather, etc.), but would still be a fun experiment.