If it is a pool based tri, then there's the cost of the pool hire. If it is open water, then there may be a cost of the venue, and there will be canoe/kayak safely people, who will cost something. Then there's the motorbike marshals, transition set-up, etc. Timing chip set-ups cost, and most races seem to use them these days. And most races like to give their volunteers something AFAIK, and that costs too.
On the other hand, I've just paid £25 for a sprint, but that is one of the Borders Sport and Leisure Trust series, and I think the pool use is subsidised. They're pretty basic, very little in the goody bag (fine by me). The last one I did they didn't use timing chips and you yelled your number to the marshals as you went through!