Zero money cost on self-pub eBook - yes, agreed that is so if you don't pay an editor or a cover artist. Interested POD is now that cheap.
In terms of
liking - beta readers that are not friends or family can be pretty hard to come by - and strangers who you know are reliable, critical, beta readers are gold dust. (It is so frustrating if someone reads your manuscript, you eagerly say "how was it" and get "alright". Um, what did you like about it? <shrug> "it was OK" etc, or the negative version starting with "sucked" - "what sucked, what can I improve?" )
In fact I am lucky and the friends and family I have are critical people with exacting standards. (Not always critical about the same things....) It is not necessarily the quality of the book I am meaning but simply whether or not it is to the taste of enough people that it will sell. Friends and family can have overlapping taste with you, and a majority of them liking the story you have to tell, does not mean it will be a popular story with other people.
So, waiting until you have finished the series before you publish it in one big hit, does leave you not knowing if a lot of people will like it. (With of course the caveat of is it not
liking, or is it
no-one noticing your series that means it isn't selling.)
Finishing the series first does have the writing advantage that if you write yourself into a corner, have a continuity error etc, you can put it right and when you do release the series, you know it is as good as it can be without the "I wish I could go back and change that".
Ray, going back to your post about having a lot of the series done except the proofing and publishing, realised I didn't pick up on your question in my last post. So, answering that - not sure how reassured I would be as a reader by your assurance. If I saw it after seeing that books 1, 2 and 3 were out and you'd met your deadlines - then maybe yes. But not on book 1. Wouldn't necessarily stop me buying if I liked the look of it, but I am not sure I'd be reassured by promises without a track record.
Also the idea that you state how many books there are to a series before you finish it - not everyone can set that in stone. It can take more books than expected. (Or even fewer....)