This does make you wonder how anybody makes any money keeping sheep, of course
Err... most people don't keep ageing wethers and expect to make a profit. They either produce 1.5 to 2 lambs per ewe, or buy animals to grow on and sell to a finisher or as finished, or as breeding ewes, within 12 months.
On your deadcart costs, if there are 10 wethers and, given their age, you'd expect them to all have died in, say, 4 years, then there would be 10/4 deadcart visits per annum. If that's the right guess for how long they might go on, the average annual bill would be £45.
On vet visits, it depends on what the protocol would be. If an animal ails, it gets the vet, then it's likely that at least half of them will see the vet at least once, as we're expecting them all to die, even if of old age, in the time period. But if the protocol would be only vet if appears to be treatable illness, then maybe less.
In general, sadly, the outcome of most veterinary visits to sheep is not good. Partly because the sheep don't show a problem until it's serious. So you probably get to pay the dead cart as well as the vet at least half the time.