Some people pay $50 bucks for a tactical bolt knob and that doesn't include the cost to install it!!
i bought mine for $30 bucks including shipping and to save money I have been researching so I can install it myself. So good luck.
I agree that $50 is a bit extreme for a bolt knob, as we only sell ours for $34.99, but you can't expect that price installed.
A basic install takes me AT LEAST .5 hours. At a shop rate of $70/hour, I think $35 for an install is quite fair considering the time spent making the jigs (hours) is not factored into that price. I'm not going to lie, there is enough margin in there to make it worth my while but I'm not going to get filthy rich off it.
Doing your own install on anything is definitely the way go if you have the skills. It is the only way to learn. But you can't really knock a business for charging a nominal amount to do work that someone else would otherwise rather not do themselves.