Valve springs & Titanium retainers

Since my 3.2 is sitting on the engine stand awaiting trans parts.
I have the bug to go through the heads. After searching around for valves, springs and retainers and wondering why they cost so much for the P-cars.
Has anyone used aftermarket ones from the likes of k-motion or Del West? Or how about the Manley Severe Duty/Pro Flo valves.
Thanks.