Is it expensive for an indie game DLC? Yes.

Does it greatly improve the game it's attached to? Also yes.

If you liked the base game (or even just the demo) and have the disposable income, ask yourself this: do you want to spend that money on something modern gaming has convinced you is okay, like loot boxes, premium accounts and/or gacha systems? Get that temporary high of adrenaline and dopamine for a little while playing the slots? Or do you want to spend it on something you know will challenge you, make you think about how to approach engineering problems while you're on the toilet at the office, build solutions to problems in new ways as you travel between planets and effectively go through New Game Plus four times yet still finds ways to improve your game 80+ hours in?

I'm happy this exists despite the price. I've got more than my money's worth and it's cemented my desire to try and get every achievement for it. I'm in it for the long haul.