The last time I flew into Buriram was with PB Air, the Nok Air connect time was never convienent. I could be in Buriram a half day before I could fly using a van.
Now when I come back from the US I am loaded down with w stuffed suitcases full of contraband (e.g., steak seasoning, Jar covers, US made kitchen tools, etc.) It would be a major hassle to move this stuff to DMK, check it in, pay excess baggage, then load baggage at the Buriram airport into a car.
Much easier to just have a Buriram VIP van meet me, load at BKK, and unload at my doorstep about 6 hours later.
However, when flying out of BKK I only have a backpack and computer bag. The convienence of flying to DMK beats out the bus for me. I don't mind the layover and I get to have a few beers and get a decent steak at Nana. Then take the skytrain to BKK.
However, this last trip I had to take the bus because Nok changed their schedule. The hassle different was not worth any saving in my opinion.
General costs:
Nok air to DMK: 1500 Baht
bus to BKK 0 Baht
Taxi to BKK 400 Baht
Bus to Mo Chit 30 Baht
Skytrain to Asoke 45 baht
Subway to Asoke 40 Baht
Skytrain to BKK 45 Baht
3+ extra hours to relax, drink beer, etc. Priceless
Bus to Bangkok 400 Baht
Taxi Ragsit to DMK 100 Baht
Taxk to BKK 400 Baht
6+ hour bus ride -3000 baht
But up to you. At my age very little is more important that my comfort.