Like anywhere, it depends on how you live. I find I spend more in Da Nang than I do in Hua Hin. But that's mainly because in Da Nang I frequently go to high quality but expensive restaurants, and Hua Hin doesn't really have any expensive restaurants worth going to.
If you are on a budget and want to live cheaply, I don't think there is too much difference between Thailand and Vietnam. Both can be very inexpensive if you try.