I was planning on going the flat bed route lol and what's cheeper? A dually axle or adapters?
The real question is, what are you going to be using the truck for? Are you doing the DRW conversion because you like how it looks or want a bit of extra stability, or are you planning to haul heavy and need the extra weight carrying capacity?
If you're hauling heavy, IMHO doing the axle's worth it even if it costs more...you have the extra material on the brake shoes, and you want all the braking capability you can get. BTW, on a similar note, hydroboost is seriously worth considering. Also, IIRC on a SRW axle with adapters, the wheel bearings aren't in the optimal location relative to the load being placed on the axle/wheels...I can see that leading to wheel bearing issues down the road if you're hauling heavy.
If it's primarily for looks, then it's not as big of a deal.
As to price...Arrowcraft doesn't have prices listed on their site, but I found a set of adapters on Amazon; they're just shy of $400.
http://www.amazon.com/Wheel-Adapter..._2?s=automotive&ie=UTF8&qid=1412029668&sr=1-2 BTW, it's worth noting that the ad (and, for that matter, Arrowcraft's site) specifically says that your carrying capacity is not increased and is limited to what the axle's rated for.
I don't know how expensive an axle's going to be...it's all going to depend on what you can find and how proud the owner is of the axle...but, again, if you're going to be hauling heavy, I STRONGLY recommend swapping the axle even if it's going to cost more.