I like the idea of a discount to pre-purchase a day pass to a lounge, because it doesn't just offer an incentive to buy the pass, like membership it brings the cash in in advance of providing the service but unlike membership it ties money spent to service provided. Nice.
My idea would be that JetBlue would set and change the level as it wanted - they would give you the option and if you decided the value of the lounge warranted it, you could pay the requisite ticket price. On Day X the price for lounge access might be a $300 ticket, but if fares moved or JetBlue wanted to, on Day Y it could be a $340 ticket. Or a $280 ticket. Or a $200 ticket. Either way they lay it out there for you to make the value judgment.