It's OEM, not retail. Fair comparison would be the price as charged by HP for the upgrade. You're talking about a retail product, so you should stick to retail comparisons. Otherwise, it's like sourcing RAM from Newegg for the PC and sourcing it from Apple for the Mac, and it creates a price imbalance.
Single driver binary != single driver
Initial support requires the same amount of work as any other product. UDA just allows for it to be rolled into a single distribution package for customers. The code and QA work doesn't disappear. A new driver distribution doesn't necessarily mean any new improvements for a given legacy product. All it means is that the driver software is still included in the package.