Don't allow yourself to imagine that the AMEX clubs would be as good if they had the level of traffic and type of passenger that UA clubs do. See how long they would be offering the better food/drink/amenities if the trays were being emptied out every 10 minutes, people left messes all over the place, and the place were packed with people who pay nothing but incur a lot of resources.
The main reason that you like the Centurion lounge is because there are few people there -- and the resulting higher quality of service that permits. If they (or UA) had to charge what that type of lounge really costs to operate I think you would be quickly turned off that idea.
Actually, I correct myself. There is a lounge that parallels the AMEX lounge, and offers you the chance to pay for that level of service -- the UA First Class lounge. And I think you can figure out what membership to that lounge will cost you.