When a self-published book has a typo on the very first page, that's two strikes. When it has errors regularly on the next few pages, that's the death knell. It doesn't always mean I'll stop reading, but it means I will not spend any more money on that specific writer.
I cannot express enough how important it is to have clean, tidy writing when trying to tempt readers to part with their money.
Even your free work should be untainted by typos because that work is supposed to persuade readers to spend money on the rest of your work. Free writing is an advertisement, a taste of what you can bring to the table. If it's slapdash or lazily edited, readers will absolutely believe that your for-sale work has the same problems.
While a small handful of readers will go nuclear at any errors, most will stay with you if the plot is good and the errors are minimal. This means cleaning up your writing as best you can. A typo or two makes it through, fine. Even traditionally published books with professional proofreaders have typos sometimes. But errors on every other page? It will prevent your writing career from blooming into something strong and sustainable.
The worst mistakes I've seen in self-published novels were consistent grammatical errors. One book capitalized all of her dialogue tags ("I'm here," He said.) and another used single quotation marks rather than double quotation marks ('How are you?' she asked. 'I'm good,' he said.), which is apparently a British punctuation thing but is dead wrong for American readers.
There are places where typos are allowed and writers can flourish despite them, and these places are casual and free and full of other stories by writers that are equally casual and free. Those places are for fun, not profit.
Everything changes when you want to charge for something. Readers demand more. They demand better. They want their money's worth.
I've seen writers who insist that the time and effort they've put into their work has earned them the right to set a reasonable price, and to that I say yes and no. You want to set a price that readers will pay. And the more typos you have, the lower that price dips. A typo-riddled work is worth less, no matter how much time you put into it. It is the final product that matters to readers, not the months and years sunk into it on the front end.
Does this seem cold? Unfair? Entitled? Sure. But we're not talking about your loving family and friends who will buy your work no matter what. We're talking about strangers stumbling onto your work and deciding whether it's something they want to spend money on. And we're talking about whether, once they've been tempted into dropping a few dollars on one story, they are pleased enough to drop more on your future work. (Preferably at a reasonable markup.)
You are asking these people to take a chance on you. To give you just a moment of their attention. And you are promising that it will be worthwhile.
Readers are not kind when they don't think you've lived up to your end of the bargain.
You want to build an audience. Build a career. You might be able do that with error-filled product if you're good at self-promotion, but your audience will not grow nearly as fast or hit the kinds of numbers your work would see if there were no errors.
Those things hold you back. They make your work look unprofessional. You might have some success through the power of your plot, but think of it like a game where you get points for a good plot, good characters, and good writing, but you lose points for poor presentation, confusing sections, typos, and bad grammar. If you have errors, you might still come out ahead in the end, but you're still losing points. And in this metaphor, the points are readers. Readers who have the money to make your writing a sustainable career.
I'm going to end with this: you already know typos are bad. I don't think a single writer thinks typos are fine. But I've seen enough error-prone self-published works over the years that I feel the need to try and push this: typos lose readers. Most times, readers leave quietly without ever saying a word. They leave, and they don't come back. They don't try again. You never get a second chance.
Every story, free or purchased, gives you one shot at making an impression on a first-time reader of your work.
Make it a good one.
