so how many lumens per cubic feet?
Lumens are for humans. A bulb can have a very high amount of lumens and be very poor at growing plants.
Plants need specific frequencies of light to grow the best. This is measured in watts. So I use watts. Fluorescent bulbs designed for plants(Gro Lux, Plant/Aquarium, Blue/Red, etc.) actually have very low luminous efficacy, as little as 20-30 lm/W. But the plant-usable watts are much higher than other fluoros producing 90 lm/W.
13 watts of CFL is around 900 lumens. This is, again, the minimum suggested. Lumens are related to the wattage by lm/W (lumens per watt), and this is termed the luminous efficiency(or sometimes termed efficacy).