After playing various browser based games it's fairly obvious that these sort of games need to try and get some sort of revenue from players who aren't willing to spend anything on upgrading their account. Whilst I'm sure the developers might not want to put adverts in their game, they might have to. Less than $5 a month could be considered insanely cheap, but I'm sure people in their minds already have the opinion that the game isn't worth spending money on. Look at the entire web, most of it is free and funded by advertising, people expect sites to be free.
It's hard for people to relate to something being valuable in a web based game, just think twice about what users are paying for - they're paying for a few extra strings of text (rows in a table/database, etc) to get some extra stats, faster experience, new items, etc. All too often these bonuses disappear after a certain amount of time, making it hard to get any attachment to the value of it. I'm sure the developer and anyone knows about developing software realises that it's something else people for (funding the developer so he/she can continue working on it), but players will see that their wonderful web game is now trying to charge them money instead of being something great and free!
Just don't have high expectations for web based games. Some might get lucky enough to pay their developers wages, some might be good enough to expand and get profit. But certainly don't spend hundreds of hours on something and expect users will instantly rush to you with their wallets wide open.