Now you might be shocked by its price, but that's where the effect comes in. At the start of your turn, its cost will be reduced by the amount of HP you lost. This will be reduced from the base cost, so it doesn't stack. As such, it's recommended to avoid using it until you have around 10 or less HP, so you won't waste more gold than you need to. If you have 3 or less HP, you reach its minimum price of 8G.
A good strategy would be to hold you money if you know you'll survive the next turn. And when your turn starts, use this, followed by Asriel Dreemurr. Just watch out for Mettaton EX, as he can ruin this strategy.
Another good use is with Innkeeper, who increases your Max HP by 5, making HG cost 5 less on your current HP than normally. Using one Innkeeper already gives its minimum cost of 8G when you're at 8 HP. Two brings that up to 13 HP. (You can also use the Health Artifact for this.)
Keep in mind that it will 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.