Their only current source of revenue is through a signature track tier where you get a certificate that is verifiable with your identity. I doubt it will stay that way indefinitely, but for now the course material is provided without charge.
I definitely foresee a subscription-type model or a la carte via the signature track model. Either way, I think anyone would be happy to pay for this awesome education.
I think you'd be surprised. If you look at the stats, there's a lot of professional-level (BS/Masters/PhD degree holders) as well as a lot of students from (broadly) the developing world. I'd submit that both those groups are probably pretty price sensitive--especially the latter, many of which don't even have credit cards in many cases.
Would "adult ed" students be willing to pay some nominal amount--say $25-$50 for a course? Probably yes in some cases but remember the pricing discussions that take place here all the time. Getting from free to paid (in any amount) is a big barrier to get people over.
Arguably the case would be different in certifications actually meant something. After all, people and their companies spend lots of $$ on various software certifications. But that's a whole other topic.
It isn't a huge barrier if the courses are as legitimate as actual university courses.
As for the pricing question, the price of a can of coca-cola isn't the same in the U.S. as it is in Sudan. Once these MOOCS establish a brand, it will be easy for them to adjusts their prices to maximize profits.
Another example, I live in the East European xUSSR country, where average School teacher's monthly wage is $200 (not a typo). But the prices of milk, meat, bread, MacDonald's, coca/pepsi, ... are the same or more, compared to e.g. Switzerland.
Last time I checked, the meat was more expensive here than in San Diego.
So not everything is adjusted by income (in case of my country, housing and renting can be stated as significantly cheaper than in Switzerland).
I genuinely wonder how they get by. (Especially with multiple family members to worry about.)
In the future, if you want people to be sure it's not a typo, you can just say the words "Two hundred dollars", then your meaning will be absolutely unambiguous.