Each extension is different, just because a domain sold for $xxx in some extension does not mean the sale can be repeated in another extension.
.so is not a ccTLD that can be rebranded into something else, unlike .tv or .co.
Look at the reported sales history for .so domains, or the lack thereof...
I think you will struggle to sell this domain. I say regfee.
I can't really think of any .so domain that could be truly valuable (at least 4 figures).