My partner missed a (revenue) flight in CE from LCY (last one of the day). She was quoted 270 GBP to change it to the later Heathrow flight (which would have taken ages, and she had to rush across London in rush hour to get it, which for various reasons was not easy for her). I bought her a new ticket instead on Avios for £10 and not very many Avios (I think about 10,000), which was done in a couple of minutes. The cash price was many hundred quid (I think 600-ish?)