"If Photoshop is for processing photos, why is Lightroom an extra expense?"
Photoshop is used for a lot more things than just 'processing' photos. It's geared a professionals across a range of creative and technical professions.
Lightroom is designed specifically for photographers (One of the team's motto is "Designed by Photographers for Photographers" as many of the folks on the Lightroom team are photographers themselves).
As such, it has a lot of workflow tools developed specifically for photographers. R & D costs, and the value that brings to the customer, is why Adobe sells Lightroom separately from Photoshop. That said, Adobe does run special pricing deals from time to time if you purchase Photoshop and Lightroom together.