You're paying for the slab of pork and the ability to start the most fun conversation in any room you're in for the following week, when you get everyone to share what they think it would taste like and then you give the big reveal of what it was actually like!
It's just good harmless fun. I'd totally pay for that.
I can't think of a time I've been to a Starbucks Reserve location and felt that the difference amounted to more than slightly bougier interior decorating. Same as any other Starbucks otherwise, and generally the coffee isn't better
Their roastery in Seattle is quite different though and worth visiting since they do have good coffee, interesting merch, interesting architecture, etc.
It's just good harmless fun. I'd totally pay for that.
Only once, obviously.