See the Berg website
here and see the price of a conversion kit. Also see the cost of a
Quaife for a VW transmission. Remember, ZF doesn't make them anymore, so he'll have to count on the stock one being intact, and not robbed away for someone's drag car. So, just the parts, minus the Quaife comes to $1819.30, because you have to have the Berg shifter too. Quaife included, the price is $2893.30. Just parts.
Installing a Berg 5 in a VW transmission isn't a weekend job. It takes more knowhow than most average, mechanically inclined men can muster. It also requires special tools, jigs, and fixtures. Figure up the labor to build one as being over the price of a stock transmission rebuild by a good margin, and a rebuild is manditory to install the parts to convert the VW tranny to a 5 speed.
See bug@5speed site
here and see the price of a nose cone. You can even buy a front mount, a Quaife to put in the 901 and be at $2231.98 for the parts to do the job. Without a Quaife, it's $679 for the conversion parts. You can even add the most outrageous price you can think of for a 911/901 shift rod, or labor to make one, and be under Berg's price. I didn't have to rebuild mine before I installed it. No new bearings, or blocker rings. It works perfectly.
Given your vast knowledge of these cars, I'm quite sure you know how it is to open up a 901, remove the intermediate plate and gears, flip the diff, and install the whole works back again with a 911/901 housing, and rod. It took me 3 hours. That's including making a shift rod, replacing seals, and relocating the vent.