Around $300 is fair, but there are deals to be had with some digging.
Maybe this page will help you talk them down a bit? Lee Enfield No. 4 Sales Data
Regarding the 2-groove rifling, I own a Long Branch No. 4 with a 2-groove barrel and it's among my best shooters. It was a good shooter with surplus ammo, but now that I'm reloading it it simply amazing and I feel will shoot better than my eyes.