Hi Fire!
First you when you use the poison the cobras will have Full Hp, once you kill them they will respawn will less HP, but the experence you get will be the same , you will get the 100% exp, in numbers 8316 XP points.
People usually use the Poison just to make the respawn easier: less HP = same exp!
Keep in mind that I had Exp Boost Activate: That's why I got 18711Exp, But Cobras Assasin give 8316 EXP at 100%.