I threw together a sampling of trips on various airlines to various destinations and came up with an overall average of $.025/mile value. This is all Y cabin as the value of J class cannot be computed from the actual fares to be paid. I might pay an up of $200 for J over Y TATL, though it could be a different case to look at QF or NZ premium economy US to Australia. Value is usually greater on TATL or TPAC, especially for UA, but it is the opposite for BA Avios redeemed domestically on AA and having that taxes and fees penalty if routed through LHR.
More interesting is that the standard deviation on that average is $.011. This means that if you can buy miles for less than $.014 you are getting a statistically (one standard deviation) cheap bargain, and if it is more than $.014, not so much.
I didn't try to do this restricted to US or to prospective value on AA.
I think I would not buy this share offer.