"hardly anything" = 80% of feed crops, foregone absorption of greenhouse gases (i.e. 32% of all of their carbon costs, cf. https://static.scientificamerican.com/sciam/cache/file/54D29... )

Is the sourcing of seaweed in sufficient quantities free of cost?

The active anti-methane compound from seaweed is likely bromoform:

https://link.springer.com/article/10.1007/s10811-016-0830-7

It would be a cheap food supplement since the methane inhibition required only about 2% seaweed in cattle food, which in turn implies a bromoform feed content of only about 35 parts per million. Though no doubt some farmers would prefer to be able to say that they add seaweed rather than a particular chemical to their cattle feed.

of course nothing is free but they think it may be feasible: https://www.symbrosiasolutions.com/team