I was once an advocate of open dirt lots over buildings, the economics have changed.

Dirt lots take a lot of maintenance to make sure they don't turn into a mudhole, and in years like this they turn into a mudhole anyhow.

The legal issue can be water quality. In many locations, a proper settling basin for manure (that does nothing for the cattle) will be required for open lots and will exceed the cost of a roof that will divert the rainfall from the manure (and can actually benefit the cattle).

The manure captured in a barn will also help offset some of the building expenses, because of increased quality and greatly increased quantity. After spreading expenses, there is still a good net value to the nutrients.

I have seen a lot of slat barns go up, all of them have cost more than $1200 and most have been around $1400 per head space when all expenses are figured in. You can go cheaper with a shallower pit, lower side walls, or no mats but you will just have other issues to offset the savings.