I'm pretty sure we had this request a long time ago...

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I think something fair would be as follows:
199 dias +1 permanent dura increase up to 105 dura, consumes 1 golden hammer
199 dias +1 permanent dura increase up to 110 dura, consumes 5 golden hammers
199 dias +1 permanent dura increase up to 115 dura, consumes 1 grand golden hammer
something else that was requested was an in-game way to +dura up to 105:
300k stone with a chance to increase dura up to 105, but has a chance to drop dura as well. dura will not drop below 70