This change I believe is not a case of unbearable costs for Google, but instead doesn’t tie in with their monetization efforts. People read the articles and they leave. The Reader web interface is very much a function over form exercise – it’s also been neglected for years, which should have set warning bells off.
Forcing people to rely more on Google+, where brands have a far greater presence and which ties in directly to search results and other services is the savvy business decision. Unfortunately, goodwill takes a huge toll – I, like many others, have been a Google Reader user for more years than I can remember, and the Reader ecosystem of third-party apps is huge.