In WDW I would expect the pricing for such a product to be in the $700-$1000 range (depending upon the time of year) per person, per day, assuming that Disney did not want to disrupt their existing FP+ system. I get to this pricing by looking at the cost of a private VIP tour which I imagine the vast majority of people do to get unlimited FP line access as opposed to having a friendly tour guide to chat to. Such a tour costs from $425 to $625 per hour with a 7 hour minimum. Let’s say the guide is getting paid $25 per hour, so that’s profit for Disney of between $400 and $600 per hour, or $2800 to $4200 per tour. Theoretically up to 10 guests can be on a tour but from seeing these tours around the parks I would say a typical group size is more like 4, so the $2800 to $4200 profit per tour would equate to $700 to $1050 per person, per day.
Of course charging such eye-wateringly high prices for fastpass line access would be a PR nightmare for Disney, which is why they dress up their existing product as a “private tour” as opposed to “front of the line access”. And their existing system also allows for plenty of upcharge events like EMM and Ultimate Nights of Adventure. They could of course scrap the existing system and go the Universal route, abolishing the 3 free FP+ and charging a more modest amount for everyone who wants FP, but I highly doubt that they will, again because of the PR hit and the massive investment in their current setup.
Also, something to bear in mind is that the Universal Express experience is not identical to FP+. Go to Universal on a very busy day and even with Express you can easily wait 40 minutes for Despicable Me and Gringotts. When I questioned our wait once at Despicable Me I was told that Express only guarantees a 50% saving compared to the standby line. This would of course be a massive downgrade to the current FP experience at rides like FoP and SDD.