What would the aprox. cost be to replace valve stem seals on 78 cb 750 ss?

Ok so I have lived with the bike for a bit now and a pattern has emerged. Cold starts she smokes like a bandit. Once the bike is warm smoke is minimal. Plugs are not fowled and power is normal. Motor acts very strong for its age. � qt of Lucas oil stabilizer is in 20/50 oil at the moment and is helping when motor heats up. Have slight oil leak through cooling fins on left side where bike lives on its side stand. Almost no oil loss due from operation (as earlier thought) except for events stated above. I believe I have a strong valve stem seal leak.