Not sure if this will help, but here you go:
Not that long ago, I repackaged and republished an old series I'd published a few years ago. I didn't make any actual changes to the content and I had that in the description and inside the book, but what I did was change the cover of the book, the title, and the series name after doing some research on the genre, I changed it all to something grittier. It was a 4 part post-apocalyptic survival thriller with 4 books (I previously had it as a post-apocalyptic adventure.)
I posted book 1 at 2.99, got maybe a couple of sales. then, I put up book 2 two weeks later and set book 1 down at 0.99 at the same time. Book 1 sold a lot of copies, which trickled down to book 2 (set at full price - 2.99). I put up books 3 and 4 in 2-week intervals, each 3.99, then tweaked my keywords a little.
Now, I didn't use any advertising, but because book one was getting purchases, and I got into some nice categories because of my keywords, I did consistently get sales across the series, with most of it coming from book 1. (And I searched for keywords by using Amazon's search tool since it drops down suggestions of most used search phrases, and I just copied the ones that matched my books. I stuffed as many keywords as I could, even using them in my subtitles). Book 1 hung around 1000 in some categories, 10,000 in others. it's dropped to around 100,000 now in its main categories, but I still occasionally make sales, and it surprises me every time. I think it's in 8 different categories, but Amazon won't always show you. I'm sure I bookmarked it somewhere, I found a website that can track all the categories your book is put into, which helped a lot with the keywords as well. The books are also on KU, I never used a free day since I'd given free copies before (over 500 - and sold maybe 20 books at the time) but the page reads were most of my income. I got thousands in the first month (around 30k from book 1 only) and it grew when I published the other books in the series. For each book, the moment it was out, I'd get a boost and sell a few copies and get a bunch of extra page reads before it trickled down.
It was a new experience for me since I'm currently working in another, more saturated genre (romance) where it's hard to make sales with just this, but there's a good growing market in dystopian/post-apocalyptic thriller.
Having a good cover and using the right keywords, is free marketing. Readers also seem to love a series. If you supplement with advertising like you're doing even on a zero budget, you'll probably make some sales but it's hard to say if it'll blow your mind or not. I sold about 100 copies total in my first two months (not counting page reads) which I was happy about, especially since I did no advertising on any of these books. (don't have the time or money to, and I wanted to rewrite the series, anyway) but I'm still getting sales half a year later and getting hundreds of page reads daily on KU. If I could advertise, I could probably make a lot more, but my mailing list (that I managed to grow up to 600 subs in a few months thanks to StoryOrigin, so you could check it out) is for a different genre entirely that I'm a bit slow to update, but it's been good for pushing my backlist.
