Let's say Lallana gets those 53 goals in 6 seasons. How much extra is that worth? Not more than double surely? That's really his true value. Anything more is overpaying.
Rather than value him by the amount of goals he scores - which is all you have done - let's try to value him in terms of importance to the team. His 9 goals and whatever in assists this season is way more than Coutinho, Allen etc. have scored/made yet Allen cost 18M and wasn't bought for his goal-scoring, neither was Coutinho who we all love to death and whose value is surely now somewhere around 30M.
Lallana's importance to the team comes in a few flavours; a like for like for Coutinho when one is injured or off-form or we need to juggle the team due to the number of matches in a week (currently we are having to move Sterling inside and change the formation, neither solution being ideal); another scoring midfielder (that we are short of); another bona fide 1st teamer and another player of high technical quality who can go past players (i.e. perfect for the way we play) and to cap it all a player who works his balls off for the team (ditto perfect for the way we play).
We need squad depth of players of 1st team quality, now. Not a paltry bench with no options. No more Aspas', Borinis or anyone else of that ilk. Do we now think that every transfer window will be a Sturrdige / Coutinho one ? Where we pick up great players for a relative pittance ? The tone of the naysayers seems to be along this line of thought. We can't afford to gamble on 1:4 chances any more, we need those 1:2 players. More likely, or just as likely, to be a success as a failure. Nothing is a 100% guarantee, there have been enough huge name failures in the PL to know that. However Lallana looks to be mould-perfect for Rodger's LFC.