Yes it is required to have an optical drive when building a new comp, because how else are you going to install an operating system. And the price range varies. Like $10-100+.
you can install windows 3.1 with like 30 floppys and then you wouldnt need a CD DRIVE...going fancy with the big work optical lol. just thought i would add my smart @$$ comment. but yea you pretty much need one.
Or just a CD drive for like nothing if you cannibalize an older computer.
If you really want to get fancy, you could get one of these
A simple CD drive would be all you need for installing an OS (or DVD drive, depending on what media the OS comes on.) There is really no point in getting a cheap, minimal drive because they are so cheap now. ($30 for a DVD burner)