At $ 200 per a month it is almost as expensive as a top tier university in South Africa.
Udacity Cost per a Month: $200
Cost at a South African University: +- $3000 per year/10 months = $300 per a month.
This excludes any discounts, bursaries etc. Also ignoring the different levels of quality and depth of what you will be learning.
It seems it's mostly the US having an issue in education costs.
Most public universities around the globe that are cheap to students are cheap because of these subsidies. Udacity, if you believe is providing a comparable good, is going the cheap-to-provide route. Admirable if it works, because we can educate the masses without spending a lot as a society.
As someone who has spent a fair amount of time in academia (masters, first half of phd), I see areas where Udacity is a good substitute, and areas where it is not. If you're an undergrad sitting in the back row of a lecture hall, why not Udacity? If you're a grad student doing research, obviously not. If you're a grad student doing advanced coursework... maybe?
For European systems, any student who is taking MOOCs is cutting their costs instead of threatening their profit model. Many students in Europe wont have financial incentives to use MOOCs for their primary area of study, but the European model doesn't really offer unlimited free higher education in what one wants whenever one wants locally.
Are the prices in the US not relative to the GDP per capita? $3k-$4k USD might be cheap in the US but that money means a lot more in different economies.