Now you might be shocked by it's price, but that's where the effect comes in. At the start of your turn, it's cost will be reduced by the amount of HP you lost. In other words, it's cost is equal to your current HP, plus 4.
You should keep hold of this until you have 10 HP or less, as it would be a waste otherwise.
A good strategy would be to hold you money if you know you'll survive the next turn. And when your turn stats, use this, followed by Asriel Dreemurr. Just watch out for Mettaton EX, as he can ruin this strategy.
Keep in mind that its cost will not reduce if you got it through Echo Flower. It will also not kill the monsters summoned due to another monster's Dust effect, like Rock's Pebble, Dimensional Box's captured monster (so you can save your other cards with it), Lamp's summon and The Heroine.