Remnant knives, that is, knives formed from scrap steel and old tools have been around for a very long time. Probably in one form or another since as long as people have been forging metals.
On the frontier, where metal was a precious commodity, every scrap was repurposed after its original usability had passed. Leaf springs and files, horseshoes and railroad spikes were common materials.
Most of that steel was untempered and would not hold an edge for very long but it would do.

These days, modern craftsmen such as Logan Pearce, Anton Malakhov, and Danny Robinson have taken this old form and transformed it into art. All use tools that they have found from various sources and re-temper the steel to make it stronger before fashioning these beautiful objects

Most of them are priced on the availability of the original materials and the amount of work put into them and sell for between $35 and $100 with the best/rarest going for up to $400.
Here are some links:

Logan Pearce:  http://shop.pearceknives.com/main.sc

Danny Robinson http://exfilesknives.com

Anton Malakhov (Russian) http://rock47.deviantart.com