I thought I would share something I hacked together -- a simple spreadsheet that allows you to spec out a furnishing item and see how much it would cost to make (or compare that to the price it is being sold at):
Furnishing Cost Calculator
Let me know if there are any problems or if you have any suggestions -- I can't promise I will add much to it, but feel free to copy and share.
Special thanks to Dominoid's furnishing database
and Reddit user Tybalt85 whose own calculator inspired me to create something I needed.