In South Korea I payed about 20 US dollars for 10 GB's with unlimited calling and mobile tv. It was a pre paid phone service.
When I was in Vietnam during 2006-2007 they didn't have any options that I know of for cellular data, although cellular service was ubiquitous throughout the whole country.
Unlimited phone service there cost about 9 US Dollars / month.
But in both countries you must keep in mind you pay for the phone outright.
