In general I would say take the estimate to replace the valve cover gasket and add .5-1.0 hours to replace the broken bolt, but keep in mind I haven't seen the vehicle. This kind of estimate really requires a visual inspection to be able to properly estimate because of all the variables when dealing with a broken bolt. Those variables can include rust/corrosion, ease of access to the bolt and thread condition after bolt removal. So you can see how without seeing the vehicle it is hard to answer these questions. Hope that helps, good luck!