You can charge them a reasonable hourly rate based on the type of work done and the skill required to do it. So cleaning can be charged at £9-15 per hour (depending on the local rate); skilled joinery work at £25 per hour.
It's nothing to do with how much rent you are losing, as advised above.
How is education supposed to make me feel smarter? Besides, every time I learn something new, it pushes some old stuff out of my brain. Remember when I took that home winemaking course, and I forgot how to drive? Homer Simpson