Reddit mentions of The Dark Forest (Remembrance of Earth's Past (2))

Dark forest page 34
https://www.amazon.com/Dark-Forest-Remembrance-Earths-Past/dp/0765386690/ref=sr_1_1?s=books&ie=UTF8&qid=1481298940&sr=1-1&keywords=dark+forest#reader_0765386690

Preface: I'm no expert on the publishing industry or Amazon or anything really, so take what I say with a grain of salt.

1. The Age of Innocence was originally written in English, so assuming that you're planning on reading it in English, Gutenberg should be fine. Gutenberg has works in the public domain, and most modern translations aren't in the public domain yet, which is why Gutenberg doesn't have the P&V translations.
   
2. The massive price difference is primarily due to backend price calculations by Amazon. If you look on camelcamelcamel, you can see just how much Amazon's prices fluctuate-- just as a random example, look at this book, whose price has gone from $16 to $6 and then back to $14 at various points in time. The main reason that those two editions you listed have different prices is that Amazon (or at least Amazon's pricing algorithms) thinks that it can sell the most copies of the Penguin edition at $11 each and that it can sell the most copies of the Collins edition at $4 each.

http://www.amazon.co.uk/gp/aw/d/0765386690/ref=tmm_pap_title_0?ie=UTF8&qid=&sr=

Paperback :)