As far as I've been told, a "beginner" writer can expect to be paid about £3000-£4000 per episode (BBC), whereas a more experienced writer gets about £6000. But then of course there's repeat fees and possible merchandise percentages to be discussed.
Plus successful writers can also make a nice little sideline in script-editing. Jeremy Dyson for instance is doing a lot of that these days. Plus there's overseas royalties if you're lucky...
It all mounts up.