Difficult to say. Why not trow "vari mu" into the search function? IIRC, there are at least four (five?) different vari mu designs in this forum, and at least two more being discussed.
For some designs you will even find PCBs offered for sale either here (White market / Black market) or elsewhere -- with other designs you'd have to build from scratch.
Concerning cost: well, leaving aside the minimum of required components and what those cost (you will find some rough "total cost" posts in this forum), it often comes down to what additional/premium components you decide to use in your build.
Why not look at schematics and BOMs to do rough calculations. Start with the most cost-intensive parts:
- case & face plate
- audio transformers
- power transformer(s)
- tubes
- big and/or exotic caps
- special pots/rotaries
Don't forget to add all shipping costs to your list! Then add a couple (or two couples) of extra bucks for generic small caps and resistors.
Another, also important consideration is how much time and effort it takes to end up with a calibrated/matched unit for stereo use.
