It's much easier to estimate if you break it down a little...The top end of the engine wears most rapidly so a top end rebuild might be the place to start. A complete piston, valves, guides, springs, ex. valve seat and gaskets pretty much covers the parts for that...plus labour of course...
Personally I encourage people to 'have a go'...There's plenty of information available and there's brains to be picked here on the forum....Not only does it cut down cost but you learn how things go together and reduce reliance on others, which as you acquire the knowledge can be quite liberating....Ian
These engines are really no rocket sience and everybody can learn to work on them.
I basically learned most of tinkering on my WD16H using the very useful contemporary maintenance and instruction manual
and the advise and help from likeminded people.
One thing I usually stress, don't go to modern (tight) tolerances. If you do it won't work as expected.
Think about the average guy of the 30ties wrt tooling and options.
They did wonders with much less tools and possibilities than we can buy nowadays.
Rebore or such "heavy" machining is best done by professionals but the majority of jobs can be done by most people.
A nice general book is "the Vintage motorcyclists' workshop" by Radco giving loads of practical tips.
If you expect a "dealer" to do the job of maintenance for you, I think you should not start with these machines at all.
I have been amazed by modern motorcyclists not being able to check their oil level!
All the practical knowledge you learn will also be handy when stranded at the side of the road!
I have never needed the AA to get me home! Some help from friends was sometimes necessary but I always came home riding.