Your friend and his neighbours just need a continuous 20 foot high cedar hedge now. Have they got a spare $40,000?
42
40k? The entire backyard 'frontage' is only 40M, and there is some existing vegetation as well.
By my estimate, that requires about 24 cedars at maturity.
To get to over 1k per tree installed........those are some seriously big cedars!
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Seriously, if they're willing to dig their own holes; it's really a matter of how long one wishes to wait for their privacy.
Small, Eastern White Cedars (not Emerald Cedars) would cost you about $50 a pop, so $200 per lot, roughly.
You might want extra to fill-out; but they'll end up seriously crowded and would fill sideways in 3-5 years no problem.
Now, that's only 5ft tall or so.
They'll add about 1ft most years, 2ft if you treat them nicely.
If you want bigger, prices do rise, substantially.
They also get more challenging for the typical person to plant.
You're probably going to run about $150-$200 per tree in planting costs for medium'ish trees (8-10ft). (plus the cost of the trees)
It really depends on how many trees, size of rootball, whether they contractor makes margin on the plants as well, warranty etc.
Or you can have 42's budget............LOL