What would it cost? Well, can you tell me what it would cost to have pants cleaned, not knowing where I live, how quickly I need them, whether there are any difficult stains, and what they are made of (dry clean vs. wash)? No, you'd probably go to or call a couple cleaners in your town and ask them. In the case of a bike it would have to be in person.

I've never worked in or managed a shop where we charged more for installing a part not purchased at our shop. But then mail/internet business is many times greater than most of the period when I worked. I still think it's bad practice. Generally one prices installation pretty generously due to there being so many unknown factors, which then gives room to discount installs for equipment purchased at the shop.

You save your money when you purchased online. You can't have it both ways.

And finally if you can't install pedals and a stem on your own you may not want to go "featherlight," in your equipment, as there are compromises there also, and you need to understand the limits of your equipment. "Price, weight, strength - choose two." (I believe this has been attributed to a bike frame builder, but can't find the source)