But tree logs are NOT dropping sap. Just wood and petals (or at least they're not in my kingdom), so it's a waste of money and energy buying logs trying to get sap. We're better off buying tress or grass. Grass drops sap.
Old log doesn't drop sap.
Tree log does drop sap (I just bought one to check again and it dropped 1 sap per two hits). If you take into account the fact that you won't need to grow it, unlike living trees, it may be interesting but very expensive!
So 300 more for a tree log (instead of a crab apple tree) and no energy cost. That's still better.