Cylinder head question

In doing a rebuild on my stock 1965 273 commando. Retired so not trying to spend a lot. Pulled the heads and a couple of exhaust valve seats look bad. I haven't taken them to a machine shop yet. From what I've read the exhaust valve seats should be upgraded anyway.
Looking at options. I know a 318 would not be that expensive but would like to keep the 273. Any idea of cost to have hardened seats installed

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