A very accurate equation to find the bottom of the barrel price is to figure the electric cost for 10,000 bbp per month and thats usually where the price will be on the exchange without forward growth. (IE the electric arb rate).
Since we are CPU mined, thats about $20 for 10,000 bbp.

It works because miners who pay $20 for something are reluctant to sell lower than this rate, therefore coins accumulate until the arb rate is at a minimum surpassed.