I believe many states get grants that cover the expenses of the testing until the grant runs out. Last year the state paid for the testing I had done and I paid a few dollars out of pocket for additional non required tests I chose to do just for my own peace of mind. You can call your state agricultural offices and ask them.
yes, I live in NH and they get a federal grant each year to do it, so it's free, just your time and labor of catching all the chickens (and guineas in our case). They do not do ducks. If you want additional testing here, it is around $1.50 per bird for other diseases. The NPIP just tests for the salmonella pullorum.