Algae is competitive at a $90/barrel of light-sweet crude equivalent, or about $3.25/gallon retail with taxes. For diesel, that's pretty good.
The immediate idea with it is to put these plants next door to concentrated CO2 emitters, like concrete plants and coal fired generators. The next generation will use strains of algae that excrete the oil so that they don't have to be harvested and squeezed, combined with atmospheric capture of CO2. Then we can shut off the coal plants.
Do not underestimate the scale of this problem. Today, the US gets 53% of it's total energy from coal. Not 53% of the electrical, 53% of EVERYTHING. That equates to hundreds of millions of TONS of coal every year. Frontline just did a special on this and the two top coal executives in the country said flat out that there is no such thing as "clean coal" (all carbon capture ideas are still theoretical), and there is no replacement for the "base power generation" (27/7, 365, throttle-able with demand) that coal provides. BTW, coal costs less than $20/barrel equivalent of oil. So dumping coal and going to algae would raise your electric bill by 4X.
We lack the political will to do nuclear right; wind, solar and hydro-electric are resource-driven, not demand-driven. Algae is great for the environment, but it will be a painful and expensive transition.