I dont think anyone really knows for sure how MV is worked out.

My guess from what ive seen is they pretty much just use the hidden stat 'current ability' like in the offline version and then an average of previous transfers of players within the same range and of the same playing postion.

Its why old superstar players are keeping a high MV because thier ability is still high even tho you would expect a 25 year old to be worth more than a 35 year old if they are both pretty much as good as each other.