I remember that before the Euro was first introduced, I read estimates on how much economical benefit every person would get (GDP per capita) once the Euro was fully implemented. Estimates like these were used in the cost/benefit analysis used for deciding whether or not it was worth the trouble of creating the Euro at all.

I guess they didn't include the risk of a Euro-collapse and the related costs, but nevertheless, it was an estimate. I do not know whether this question is too speculative, but it is truly the most interesting question I can think of. If it would be too difficult to estimate for the whole world, then an estimate for a region would also be interesting.