Sorry to run on, a correction is needed.  A much better approximation 
requires that the rule-following minority finds the next TWO blocks, so 
the cost is

(total miner revenue of block)*(fraction of hashpower following the rule)^2

So the lower bound cost in this very pessimistic scenario is .0025 BTC,  
still quite high for one transaction.  I guess miner could try to make a 
business out of mining double-spends, to defray that cost.

On 5/11/2014 9:41 PM, Tom Harding wrote:
> Back up to the miner who decided to include a "seasoned" double-spend 
> in his block.  Let's say he saw it 21 seconds after he saw an earlier 
> spend, and included it, despite the rule.
> The expected cost of including the respend is any revenue loss from 
> doing so: (total miner revenue of block)*(fraction of hashpower 
> following the rule).  So today, if only 1% of hashpower follows the 
> rule (ie a near total failure of consensus implementation), he still 
> loses at least .25 BTC.
> .25 BTC is about 1000x the typical "double-spend premium" I'm seeing 
> right now.  Wouldn't the greedy-rational miner just decide to include 
> the earlier spend instead 

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