How much should a friend charge to fly?

Long story but some financial doldrums I've experienced for the past year
look to be coming to an end. I sold my way-too-big house and am getting
into something less insane. Business is recovering and, I dare say,
rocking. My purdy wife still loves me. Life is good.

More importantly, soaring into the wild-ass yonder is again in my future.

I've got a buddy who has the same model Cherokee 180 I once owned. We've
been going up every now and then and sharing wet costs. He's offered to put
me on his insurance as long as I'll pay the difference between what he and
his brother are paying now and whatever it will be when I am named on the
policy.

So what is that cost difference likely to be?

Second question: He doesn't fly much at all. Perhaps one a month or so.
Taking into account that I will be selflessly devoting time to keeping his
bird well-oiled and used to flying, what's a fair price to pay him per Hobbs
hour?

--
Jim Fisher