Make sure you use genuine Sue Chef (tm) brand Tree Poison, available from finer vendors everywhere or direct from me, postpaid. "When it's gotta be dead, call the Chef!"
Why buy quality poison? At Lord Bacon-o's Poison & Pies, we just take a bunch of toxic junk and mix it up. Does it smell good? No. But it's darn cheap!
Current auction prices are running around 1000c a bottle, which is 200c a dose. I seem to remember old-style tree poison sold from the vendor cost over 300c a bottle, so that's not so bad.
Demand is low. There was a period of time when people pursuing certain tree badges were throwing money into the tree poison market. Not so much any more. :(
You remember wrong - old tree poison was 100c per from the gardening vendor, while antidote was 20c. However, each dose took up an entire inventory slot, which was very annoying. You pay for convenience I guess