Cells shouldnt just disapear when you use them. They should turn into empty cells that have to be refilled again. That way making bio fuel only requires a investment of tin when you are setting it up. Also bio fuel should be alot more efficient than burning cactus raw more investment = more reward. Ofc cells can be made more expensive if this happens.
Still cactus farm can be quite nice for powering your buildcraft steamengines through making a redstone engine of the same size will produce alot more. My farm was over 50 by 50 blocks and had several floors (5-6 floors or something) and made enough cactus to keep 8 steamengines running. Better use for such huge amounts of cactus is recycling through which we eventualy did. Be sure to use a obsidian and diamond pipe to suck up all those useless items you get from it :). With recylcing you also get a large part back in energy in the form of single use batteries along with tons of diamonds.