Does Wade & Gatton still do wholesale trees? I bought some from them years ago. I've seen a few websites listed for them that are dead addresses (wadeandgatton.com linked on their FB site and
www.wadeandgattonnurseries.com). FB doesn't seem to indicate they do.
Anyhow back on topic...you could go that route and use Cost Compounding Technique. But, if they get "biggest possible" that MAY be different than the "largest commonly available nursery plant" (pg. 60 of The Guide, 10th Edition). Are 8" caliper trees "commonly available"??? The previously mentioned Ohio publication says 3.5". You can demonstrate otherwise, but have the data to back that up.
Having said that, if they go that route as you described, and the neighbor is happy, then you don't need to appraise anything...the case is settled with 2 parties coming to terms based on new trees being planted.