How much would I have to pay to replace the freeze plugs of a 2008 Ford Focus?

Answering your questions: Replacing any and all freeze plugs is typically a "flat rate" repair, meaning you pay whatever time it takes. There is no real standard. Consult a local repair shop near you for guidance. How many are there? Unknown, however, seek guidance from a local shop to pressure test the engine and seek where coolant leaks from. Most times you can replace plugs with access but not full removal of the engine.