I'm not sure how much the battery life is affected by run time - the unit is permanently on (displaying hours run) even if the outboard is not running.
I would get a price direct:
Tiny-Tach: Contact page for ITCO
Richard Dunn is a really helpful bloke - when my TinyTach cable broke after a couple of years, he sent me a new cable.
SIB (lack of) performance seems to be a popular topic - when you look at the costs of outboards / props / mechanic's time, £60 strikes me a s a good investment.