beer is roughly $1000/ton (based on a 150lb keg costing about $75). You're looking at maybe a 5% rise in cost.
Except you're looking at the weight ratios wrong. Producing 1 ton of beer produces less than 1 ton of spent grain (beer is mostly water). So the ratio would be probably be closer to.... I don't know 1:5. Which would only be a 1% rise in cost.