Poison is a 3rd level druid spell. At the minimum DC of 13, it would cost 5 x 3 x 2000 or 30,000 gcs. DC 14 would be 33,000 and each additional DC would be +6k. Black Lotus extract has a DC 20 so that would bring the price to 69k but it does 3d6 instead of 1d10 which is 10.5 avg damage vs 5.5 avg damage for poison so you'd need to double empower the spell for +4 spell levels which raises the cost to 13 x 7 x 2000 or 182k but that has DC 19 so you have to add +14k to get DC 20 (14x7x2k) for a final price of 196k for an endless supply of black lotus extract item. (alternatively 50x the one use price is 125,000 so the magic item price is in the ballpark of the unlimited = 50x single use rule of thumb)