From what I’ve seen in similar situations, the best way to get an accurate value is to ask an arboricultural consultant to do a tree valuation using something like the CAVAT or Helliwell method. They’ll look at size, condition, amenity value, etc. If the timber has already been removed it’s still possible to estimate based on stump size and photos.
Replacing them with mature screening trees can get pricey depending on size, even semi-mature stock can run into the hundreds per tree once you factor in planting and aftercare. Fully mature specimens can go into the thousands. Might be worth getting a couple of quotes locally to compare.
