I wondered: if I were responsible for meeting the food requirements of a robin, approximately how much would it cost per day?
A robin weighs around 20 grams and the usual estimate is that he or she needs about 40% of body weight per day. Mealworms (a favourite food) are about £6 per kg, so a robin's daily requirements cost about
20 x 40% x (£6 ÷ 1000) = 8 x 0.6p = 4.8p
or 5p in round figures.
The figure of 40% (sometimes 50%) of body weight is often quoted (because it's surprising and illuminates how much work small birds must do to survive) but I'm not sure what study or studies it originates from. I tried to find out but didn't really get anywhere. However I did hit on a very interesting paper from 2019, that examined how people feeding wild birds has over the years shaped both the number and variety of birds visiting feeders. The British Trust for Ornithology website gives this overview of the findings:
Newly published research from BTO shows how the popular pastime of feeding the birds is significantly shaping garden bird communities in Britain. The populations of several species of garden birds have grown in number, and the diversity of species visiting feeders has also increased.
The paper was published in Nature and is freely available here.