Purplepandas - do you mean for BA, Masters or Research degrees? Have done all three and, yes, it is quite a bit of extra work.
I find that examining a PhD takes the best part of a week by the time you've read it (and possibly had to do a bit of checking of sources and other related material), travelled to the other institution and written your preliminary and final reports.
For taught degrees, it partly depends on the size of the cohort (though for larger cohorts, you'll be sampling rather than reading everything). The money is definitely not a reflection of the time spent (in fact, my department has approached people to examine the BA who have quite clearly stated that they're not prepared to do it for so little money). I am currently drowning in MA theses, as a massive pile has turned up from the institution I external examine for, at the same time as I have to mark my own students'. Often the timing of the external examining work is awful!
It can be a good experience, though. For taught degrees, I have been fortunate to work with lovely departments, and it's been nice getting to know their academics. It's a good thing to have on your CV (particularly PhD examining). And it can be useful to see how assessment works in other institutions, and beneficial for both parties to share ideas. For PhDs, of course, it's always good to see what is happening at all levels in your field.