Actually I think the 450 Bushy upper has limited value to Indiana residents as it is not legal for deer hunting. In the same light, 458 Socom uppers from Rock River Arms are priced between $650 and $700 and that round is legal for Hoosier deer season so I'd think that to Indiana residents the 458 Socom has more 'value' than a 450 Bushy.
I'm guessing it is used. I don't know what the discount is for used. I know I see the 450 Bushy uppers for $750 to $790. I see complete 450 Bushy guns for about $1100. That places the value of the lower at roughly $350. Again that would be a new gun price.
With the rise in demand and the rise in prices it seems like complete ARs are up about $100 to $200 right now . . . if you can find one.
Just a month ago a built Bushmaster lower would sell for $285 to $315. Now they are impossible to find for sale and people would probably stand in line all day to buy one for $350.
Honestly I have no clue if you'd be better off selling it in parts or as a whole gun. I suspect that parting it out would yield more money, but would require more effort too.
You could probably get good money right now for the lower.
Bushmaster had a fall sale almost a month ago and 16" and 20" complete .450 uppers were on sale for $500. They "MIGHT" have another Christmas sale near the end of November.