Anyone who ever filed for green card will confirm that an immigration attorney charges a few thousands for the green card process, not more. The cost to a large company, who has one on retainer anyway for H1-B, is obviously less. Something tells me that foreign employees were underpaid much more than the cost of the green card.

For the employee the situation looks more like this: https://rachelbythebay.com/w/2018/09/08/visa/