I have always planted the brassica seeds from my local co-op. Two of the most widely available in most places is Purple Top Turnip and Dwarf Essex Rape. Either one or a combination of the two works well.

1.5 pounds will plant a 1/4 acre, no more than 2#s total IMO. Both should be less than $4 a pound, maybe much less. PTT is $3.35 a pound and DER is $2.10 a pound around here.

It's gettin a little late for brassicas, you may want to consider Winter Rye, Oats or Winter Wheat to get the maximum amount of forage planting now.

25 to 50 pounds of the grains per 1/4 acre is sufficient. None should be more than $18.00 per 50 pounds, feed oats is $8 per 50#s here.