But in the publicly visible (longest, official) chain, can't I estimate the effort that was used to mine from a certain transaction up to the head? Say the transaction I want to hide is b, then the attacker would need to mine a longer alternative chain (c', d'...). If transaction b was for $1000 and we can estimate that the mining of the visible chain c...f is at least $2k with a conservative estimate, can't we estimate that the attackers chain c'...g' must cost something at least vaguely proportoinal to that as well? Is it not even possible to make a conservative guesstimate of cost-vs-chain-length?

    a<-b<-c<-d<-e<-f
     \
      c'<-d'<-e'<-f'<-g'