I wont go into this "what is Ray worth in rebuilding team vs in contender", cause I can't stay objective when my favorite player (since UConn days) is in question, but I will state few facts.
NBA is league of overpaid "pieces".
Rays home is in NE, he is taking home discount for sure.
There is no SG will his skillset for the price we re talking about (other top SGs are paid accordinglly, top SG wannabees are overpaid).
The thing is if we end up getting Max contract FA, we will have enough money to sign both Ray and KG on a hommer deals.
If we don't get max FA and go full rebuild mode, it will depend more on Ray than on Cs (weather he will want to chase another few rings with a contender, or build his legacy to the end in Boston, probably earning him self retired number while schooling younglings).
Big Baby is playing for a 6.25 mill/yr, and you want to replace Ray if he asks too much? In either case there is cap limit (upper for contender, low cap treshold for lottery bound teams), and I d rather end up paying $$ for Ray than for some rookie/veteran who is not worth it.