It's often not cost effective to get the vet to an individual animal, but you have to look at it as an overal cost to the operation. If, over a year or two, the vet bills are too high, then you need to address the issues that are necessitating vet visits. And if you can't afford a level of vet visits, then you can't afford to be keeping sheep. I'm sorry, but that's the way it is.
There are lots of ways to minimise vet charges, however. Telepohone advice, as has already been mentioned; taking the sheep, or especially lamb, to the vet rather than getting the vet to the holding; asking the vet if they might be able to call in while passing - sometimes you can get a reduced or waived callout fee; taking courses on various aspects of husbandry and welfare, so that you increase your ability to diagnose and treat problems yourself, and, as importantly, you are better able to communicate to the vet over the phone, reducing the number of callouts that are needed.