Nifty little tool showing the estimated cost of controlling the
   majority of the Bitcoin network:
   [1]https://www.resallex.com/bitcoin/brix
   Would love to see something similar for the Tor network - my guess is
   that the cost there is probably at least an order-of-magnitude lower,
   but that's just my intuition.
   R
   From the site..

Introduction

     Equilibrium 51% Attack Cost: This is a metric attempting to
     calculate the total present value cost required to attack the
     Bitcoin network through majority hashing power (51% attack). The
     metric is meant to be viewed as a snapshot in time as if an attacker
     decided to invest in attacking the network under the current
     conditions.
     BRIX Score: "Bitcoin Robustness Index" - The relative rank of
     Bitcoin's 51% attack cost compared to annual military expenditures
     among all nations.

Method

     This metric is, in essence, equal to 51% of the present value ("PV")
     of all future revenues derived from bitcoin mining using current Mt.
     Gox prices. Revenues include both block rewards and transaction
     fees. The purpose behind using PV as a measuring tool is to
     approximate the incentives to miners to build upon the Bitcoin
     network. The measure can be viewed as an aggregate of all the
     cost-benefit analyses done by individual miners. We believe this is
     superior to other methods of calculating the attack cost, including
     variables such as current hash rates and current capital costs,
     because the model is independent of technology advancements. Under
     the equilibrium model, miners will continue to invest in equipment
     until they reach the point where marginal cost equals marginal
     revenue (the point of profit maximization). Under perfect
     competition (of which bitcoin mining is effectively), this point
     will also be where aggregate cost equals aggregate revenue. If we
     assume the variables that can affect mining revenue are held
     constant ($/btc & transaction fees), then it is easy to calculate
     aggregate revenue and therefore also aggregate cost. Since we know
     aggregate revenue equals aggregate cost, by calculating 51% of
     aggregate revenue we effectively calculate 51% of the aggregate cost
     to miners.
     [Economics_Perfect_competition.png]
     We calculated this metric by discounting each block reward (210,000
     blocks) as if it were an annuity and then discounting it further to
     its present value. Then, we added estimated transaction fees based
     off historical records.
     [101503_7.gif]

Assumptions

     This metric is meant to represent a model at equilibrium. Therefore
     it represents a snapshot of 51% of the incentive to miners at the
     current price and current transaction fee levels. The idea is that
     miners are willing to invest in the network as long as it is
     profitable to continue doing so. We assume the following:
     * Rational Actors: We assume all mining participants are rational
       actors and strictly pursue profit maximization. We ignore all other
       motivations, including political, emotional, and reputational. All
       other heuristics and biases are ignored.
     * Static Variables: We assume that the variables in the model are
       static, and therefore represent a 'snapshot in time'. There are no
       growth forecasts for either price or transaction fees.
     * Perfect Competition: We assume that all miners and potential
       attackers have access to the same technology, resources, and
       information. There is no technological advantage for any party that
       would exclusively decrease mining costs or otherwise acquire mining
       equipment faster.
     * Discount Rate: 8% Our model discounts future cash flows by 8%.

References

   1. https://www.resallex.com/bitcoin/brix