One of the only pain in the ass aspects of living here is that unless you have a job or are married you are going to have to jump through a lot of hoops as far as staying here is concerned. I haven't lived in very many countries outside of my own but one of the things that I do like about living here is that they don't really seem to mind how many visas you get, you just have to make sure that you actually get them. The last visa run I went on was just like the one that so many other people do and that was simply to get on a plane to Bangkok, hang out in the airport for a few hours and then fly back. It takes half a day or a full day if you are unlucky as far as the flight times are concerned but the route is used enough that it is kept relatively cheap.
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The good news about visa runs using Thailand is that as long as you are from something like 70 countries that are favored by them you don't have to do any paperwork. You just stand in line, wait for the immigration window, hand over your gear, and unless you get a super-moody official - which does happen - they don't even say anything to you. I have only been asked a few questions by officials and they are probably just protocol when they ask "how long are you staying" or "you have many Thailand stamps" in a rather accusatory fashion. The fact of the matter is that they HAVE to let you in because there are no official rules about consecutive stamps to Thailand on tourist visas. The only thing I can imagine is that they feel somewhat obligated to flex their authority muscles since they basically have you at their mercy while you are in those long lines.
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It can be a little bit of a pain in the butt because depending on which airport you fly into there could be a ton of people in line waiting to get in. If you have a choice and are doing a visa run to Thailand I highly recommend that you use the older Don Muang airport because there are far fewer international flights that come and go from this older airport and therefore you are going to spend less time in line like a cow waiting for your chance to feed.
The other airport is more modern but in my opinion it is just a gigantic shopping mall that happens to have flights coming and going from it so you just spend a bunch of time walking endlessly to get to your gate. I guess exercise is good.
The flight itself from Da Nang anyway, is about an hour and a half and these flights don't suck any more or less than other flights. If you want to make your journey a little bit more comfortable, when you are checking in ask for an exit row. As far as I know they are not allowed to sell these seats ahead of time because you aren't allowed to sit there if you are too old, too young, or traveling with children. Being in these rows gives you something like an extra foot of leg room and in the dozen or so visa runs that I have done, I have had entire rows to myself because nobody else apparently asked for this option.
Air Asia doesn't charge extra for this but Vietjet does. I don't know how they get away with this but that is just the way that it is.
At the end of the day you are going to be paying about $125 for the flight and the visa back into Vietnam and $0 for the visa to Thailand since they have free 30-day visas on arrival. Again, check to see if this applies to your country. Unless you are from somewhere weird there is a very good chance that your visa will be free.
From Da Nang the entire trip takes 10 hours if you like a stress-free trip, or it takes around 6 if you take the flights that are closer together. I find the 2nd option to be too stressful since if the flight departing Da Nang is delayed at all, you could be in big trouble once you arrive in Bangkok. For the most part Air Asia does a good job of being on time so I think the 6 hour one would be fine for most people... I'm just a nervous traveler and would rather be in a chair reading a book for an hour at the gate than be rushing to get there on time.
So the overall cost of legally living in Vietnam per year is around $1500. Compared to other options, this is actually quite cheap.