it should be judged by the amount of effort put in and not necessarily the financial worth of the job?
Someone did us some drawings for some work that we are doing on our house and it probably took a couple of hours at most but was worth about £400-500 (structural engineering).
Now they are asking me to do some work which is physically demanding and will take me about 4 full weekends to complete. I'd also need to get childcare. The finished result would be about the same value as the work they did iykwim. I'd rather pay them for the work they did to be honest, I'm trying to worm my way out of it but would that just be rude.
I also have a 12 week old and have returned to work this week so I'm knackered.