The PMDG planes are hardcore system simulations. I have one (the 737NG) and have just started delving into it.
If you're like me and you casually fly a dozen-plus different plane types every month in sims, these planes don't make sense and don't have good ROI.
But if you like to fly P3D/FSX as a full-on system simulator, learning all of the switchology, setting up flight plans in the FMS, etc. the authenticity and detail in the PDMG 747 duo is probably worth every penny. If you're going to fly a plane for maybe 5 hours tops, $200 is stupid. If you're going to spend all of your sim time becoming a master of one plane type, and want it as realistic as possible, that $200 isn't as crazy if you're going to spend 200+ hours flying it over the course of a year. I mean, the darn thing even swaps cockpit systems realistically depending on the airline you're flying for.
So yeah, I have their 737, and the Aerosoft A321 so I'm good for deep-dive fully simulated tubeliners for now. But for some, it's probably worth the investment. They'd probably make more money at a lower price, but that depends on whether the support costs per user are high.
The only thing I'd criticize here is making the 800 only available as an add-on. If I ever did want to buy another hardcore tubeliner model, I wouldn't need both the 400 and 800, and I'd never spend $200.