I've got a sole proprietorship, no employees, and also have a sub-contract that requires workmans comp. In California, there is a state run fund which costs $225 per year just to have the certificate without any employees. Its cost is factored either in as overhead or as part of my hourly rate.

In my case the prime contractor passes all of his contract requirements down to his subcontractors.

The requirements will vary from state to state.