Westerby, not that I would want to make this information generally available, of course,

but from my observations I would say property prices in Nong Khai are definitely cheaper than in Udon. Understandably I suppose, because NK doesn't (yet?) have anything near the range or number of shopping outlets, facilities, services etc that Udon has, and probably won't for a long time to come. NK residents often find themselves having to go to Udon to buy furniture or more specialist goods. For example, Udon has Index, Home Pro, Carrefour, Big C as well as a much larger Tesco, and many large Builder's Merchants and DIY type stores. So yes, property prices are cheaper in Nong Khai, like for like.
The above said, Nong Khai has a range of small hardware shops, a couple of Home Marts, a Tesco that we've already spoken of, and a number of furniture shops, - as well as Tha Sadet market for the more obscure or unusual items!
You will no doubt have seen the news report/thread elsewhere on this forum about the developers of UD plaza and shopping centre in Udon, are looking at the idea of a similar project here in Nong Khai.
Re the prices of vehicles, I would suggest that new vehicles are much the same price wherever one goes in Thailand. With secondhand vehicles, the range and numbers available are understandably lower in NK, which may (and I'm only guessing here) result in prices holding up well (or even being a bit dearer), as compared with bigger cities like Udon or Bangkok. So if you are looking for secondhand, whilst your choice will likely be more limited, maybe with less scope for discounts, you would however save the time trouble and costs of buying elsewhere, having to transport the vehicle back to NK, re-registering it locally etc.
All the major dealers are represented in Nong Khai, if you are thinking of a new vehicle.