well break it down-
at this time you can buy new Solo batteries on B&H Photo for $30/each.
some guys on this forum have successfully purchased gopro hero 4 blacks for $129 (refurbished units from gopro )
and your Solo- comes complete with Solo,controller, battery for Solo, battery for controller, chargers for drone and controller batteries, leg extensions and props plus the fixed mount. Your upgrade to open solo could be considered “ sweat equity “ but not sure it has an added value. So here is my opinion-
Solo complete in pristine condition- $200 - $225
Additional battery- $20 -$25
Gopro Hero 4 - pristine- $175
keep in mind, it’s only worth what someone is willing to pay, and on ebay, there are plenty to chose from. This totals out around $400 and one can find a Solo Gimbal Backpack for $425- $450. Guys will find the gimbal more appealing then the gopro.
Also take into consideration depreciation, used Solo loses value especially without the 3 axis gimbal- ( this is what everyone is after)
Hope this helps and I’m sure others will reply.