The VISA network (VisaNet) supports 4.4 Trillion dollars of commerce per year (2009). If we assume (somewhat arbitrarily), that the Bitcoin Days Destroyed metric approaches 50% (half of all coins are used every day, on average, to facilitate transactions), then the value of the Bitcoin required to support 10% of the transactional volume in the VISA network (once all coins are minted) is $4.4T * 10% = $440B / 365 days/year / (21M BTC * 50%) = $114.80.
Does this approach seem valid? 50% BTCDD is arbitrary. 10% of the VISA network, is arbitrary but seems like a reasonable goal. What needs to be changed to improve this estimate, and what would a better way if any to compute the imputed value of the BTC based on its use as a transactional currency, rather than a speculative store of value?