Cost estimate? Or if I have to ask...
That depends on what you are looking for.
The bottom bracket requires a longer spindle for room for the second bearing. So, it requires a bearing, lengthened bottom bracket, bearing spacer, sprocket adaptor, crank arms, chainrings, and freewheel. The freewheel can be a basic flanged freewheel or a White Industries flanged freewheel. I would guess it will be maybe $250 or $300? I have to total up all the parts that go into this and get back to you.
1% of the world's population can think "Outside the box". The rest are firmly stuck within the box. Where are you?