There is certainly a price difference between an as-issued or even a refurbished Romanian and a mixmaster like you described. If I saw something with a modified bolt carrier on GB, I would likely classify it in the Bubba category. As a shooter it's probably just fine, but the collectable value is pretty low and it would only fill a spot in a collector's collection until a nicer specimen came along.
Now is the spread between a bubba shooter and a refurb romanian $300? I'd guess closer to $100-$150 myself, but that's where all the criteria that make any particular gun valuable come in. There are lots of different criteria that determine price: rarity, arsenal condition, current condition, that particular type of gun's reputation, how it fit's into a collector's niche, and finally what part of the country it's being sold in. Also note that we only have a sample size of two guns this month. Hopefully more Romanians show up in April and the numbers become more representative of reality.
I agree with you though, I think the $580 gun was extremely high. The $426 gun I think was right in line though.