Undying Mercenaries by BV Larson is my goto sci fi recommendation these days. 15 books out now.
And the first 14 are out on Audible. Narrator is really good.
I found this page a while ago, but most people don't seem to know it exists.
https://www.audible.com/ep/FreeListens
It's a bunch of free books available on Audible
depending on how you define cheap, you can get Kindle Unlimited for .99 for 2 months (deal expires at the end of this month)
https://www.techbargains.com/deals/amazon-kindle-unlimited-membership
Once you have that in place there are a ton of great books for 1.99, here are the ones I've gotten:
You can probably see from my list I enjoy Sci-Fi... I have several other series that fall into the cheap KU list that I haven't gotten yet.
If you want to pad your library a bit, you can always look at signing up for AudioBookBoom, there are usually some self-help/similar style books that you can listen to, you get the credit to buy it, but you can usually buy the book for 4-5 dollars. So if you just buy the book and keep the credit it's like getting a credit for 4-5 dollars.
Undying Mercenaries would like to have a word.
Theyre fun books if you want pulp space opera adventures!
Not exactly strictly space based series but I really enjoyed the Undying Mercenaries series by B.V. Larson:
https://www.amazon.com/Undying-Mercenaries-Series/dp/B074C7NGNX
Red Rising by Pierce Brown was pretty cool too
https://www.amazon.com/Red-Rising-Pierce-Brown/dp/034553980X
They're both more scifi war books the feature advanced civilisations and space combat.
If you're into more comedic stuff, the Bobiverse by Dennis E Taylor series was great also. It has more exploration elements but still has some action.