Over a 6 month period in 2016 I did London to Beijing by train. I blogged/tweeted the The Trans Siberian leg of it https://tombh.co.uk/tweets-from-trans-siberia
How much was it. From your tweets, looks like most people were friendly, if I was to do this, I think I'd rent an entire room though.
I don't actually remember, I think it was something like $250. And that was through an agency, so surely it could have been even cheaper if I understood enough Russian to buy the ticket directly.
There are first class rooms as well, for even more comfort and privacy. I can't imagine they'd be bank-breakingly expensive either.