The cost of the parts to replace and engines valve guides ranges from $30.00 to $100.00 and then it can take around 2 to 4 hours for a mechanic to replace the valve guides.
So if you hire a shop to replace the valve guides it could cost between $200.00 to $400.00 for parts and labor if you hire a mechanic to replace the valve guides.
If you don't know how to replace the engines valve guides then you should hire a mechanic as it takes some knowledge to replace the valve guides and requires removal of the engines heads and you also need to replace the head gasket as well.
Since you have the heads off and the head gaskets off those need replaced because the old head gaskets cannot be reused.
You'll also need to replace the valve cover gaskets as well so factor in those costs.