Considering Liverpool paid €20m for Borini, Roma paid €20m for Destro, Inter paid €20m for Icardi and Dortmund €20m for Immobile I only think €20m is a fair price for Morata. Mind you his last season with Real Madrid B he was the top scorer and he has managed to score 9 goals in the first team with RM. I think Morata has all the abilities to be the new #1 choice striker for the Spain national team in a few years..
I don't know, Morata seems highly unproven, he barely played for Madrid. 20M for an unproven kid seems to be too much. Kinda weird, since he is a CF and will fight Llorente for a starting position.
In the end the buy-back clause will only be relevant if Morata actually wants to leave for Madrid which probably means they're planning to play him regularly and that seems highly unlikely to say the least - even if he destroys Serie A over the next few years.