I run an eBook Store that has 400,000 "pay" titles and the entire Google Free Library. Generally, the Google books are decent in quality but not perfect. This is because, I think, they are PDF. PDF is little more than a picture of a printed page. So, any errors in the origional go unfixed. This is a problem and the industry and the IDPF (International Digital Publishing Forum) are working on this. Largely, when you take a photograph, some of the text can be missed or simply cut out. I have purposely read some really old classics like "The Hunchback of Notre Dame" and found some errors. YET... reading the eBook made from the origional manuscript, after seeing all the versions of the movie, brought the characters to life for me and SO many details were filled in. Was it ideal and well proofread and typeset... not totally. Did I enjoy every minute... absolutely.
This eBook Store of mine (
www.EZread.com) also has many works by Independent Authors. We really go after Independent Authors and Publishers and WANT to get their eBooks up and selling. Many of those are $1.99-$5.99 and some free. Steven Lake, for example, does a wonderful job. On those the quality is high as the authors really take pride. So, in my opinion, price itself is not always a good indicator of proofreading.