Alex Tabarrok, quoting DeLong, says:

Cost to buy and free all the slaves? $90 per capita.

Yes, but the institution of slavery would still have been in place. The people to whom the $90 had been paid to would find a way to buy more slaves. I know that the transatlantic slave trade had been illegal since 1807, but smuggling was ongoing and there is no way the skyrocketing demand wouldn't have led to an upsurge in smuggling. So it's not clear what the $90 would have bought.