One had might as well do it right. There are other variables, although they vary less rapidly over time than does difficulty:
(Difficulty hash/block * Rate $/hr) / (Hostrate hash/sec * 3600 sec/hr * Reward xmr/block * Nonorphan block/block * Exchange $/btc) = N btc/xmr
Rate (EC2 c3.8xlarge spot instance pricing) is ~0.2575 usd/hr [1]
Exchange is ~600 usd/btc
Difficulty is 8.1e8 [2]
Hostrate (EC2 c3.8xlarge) is 610 hash/sec [2]
Reward is ~16.07 xmr/block [3]
Nonorphan rate (pool mined) is ~0.93
1. http://aws.amazon.com/ec2/purchasing-options/spot-instances/
2. https://minergate.com/blockchain/mro/block/372ad817b077c058e269ae0ea7c6d6bc603578be568e3f51f2e4d436ede04c51
3. https://docs.google.com/a/southoftheclouds.net/spreadsheets/d/1MI-ic0Os25hgGUImW54sUIjZY_pUNQNa_W8Se5pRGBs
EDIT: with new parameters at this moment the marginal production cost estimate is 0.011 btc/xmr (2014-06-23 01:00 GMT)
thanks for the detail, i was trying to figure this out myself.
if this is the case, why is anyone selling Monero this cheap?