Another option is to turn around evoke.
Mercenary Poisoner
Creature - Human Assassin
Contract
(When ~ enters the battlefield, sacrifice it unless you pay .)
When ~ enters the battlefield, destroy target nonartifact creature.
2/1
Or, shift the cost to the upkeep...
Mercenary Poisoner
Creature - Human Assassin
Contract
(At the beginning of your upkeep, sacrifice ~ unless you pay .)
When ~ enters the battlefield, destroy target nonartifact creature.
2/1
Or, shift the effect to a dies trigger, so you actively want it to die, but make people pay for the contract or it won't work... (Though now we're moving into territory where you need an outside card to make it work on command.)
Mercenary Poisoner
Creature - Human Assassin
Contract
(When ~ would be put into a graveyard, exile it unless you pay .)
When ~ dies, destroy target nonartifact creature.
2/1
Maybe word that in another way? (What's this about reflexive triggers?)
Mercenary Poisoner
Creature - Human Assassin
Contract
— When ~ dies, you may pay
. When you do, destroy target nonartifact creature.
2/1
Or maybe there needs to be a cost to keeping the creature around? Maybe we should reuse upkeep?
Mercenary Poisoner
Creature - Human Assassin
Upkeep
(At the beginning of your upkeep, sacrifice ~ unless you pay .)
When ~ dies, destroy target nonartifact creature.
2/1
Or maybe what you really want is a marriage between evoke and upkeep?
Mercenary Poisoner
Creature - Human Assassin
Contract
(When ~ enters the battlefield and at the beginning of your upkeep, sacrifice it unless you pay .)
When ~ enters the battlefield, destroy target nonartifact creature.
2/1
And then maybe switch it to a dies effect?
Mercenary Poisoner
Creature - Human Assassin
Contract
(When ~ enters the battlefield and at the beginning of your upkeep, sacrifice it unless you pay .)
When ~ dies, destroy target nonartifact creature.
2/1