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.
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. It will also allow it to reach its minimum cost of 5G (when you're at 1-5 HP), as without Innkeeper, its lowest cost is 6G. With 2 Innkeepers, it will be at 5G at 10 HP, so you can consider using 2 Innkeepers in your deck.
Innkeeper is also the only card that makes HG's minimum cost of 5G be worth noting, as normally its cost can't be lower than 6G due to you needing to have 1 HP to make it that cost. Innkeeper will increase your Max HP by 5, though, so without that restriction, HG would be able to get lower, even reaching 0G with you at 1-5 HP after using 2 Innkeepers.
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.