Why would someone say that there is no average? There is. There is a mean pay and a median pay. The only complication is that there is a lot of variability in the pay distribution.
Most guys with a pro record have other jobs to support themselves.
While it is true that there is an arithmetic average, because of the huge disparity in pay that you reference yourself, it is a meaningless number that does not represent anything resembling a true average.
A meaningful average would be in a bell distribution, the average representing a figure close to what the majority of fighters actually make. This is definitely NOT the case here.
However.... rephrasing the question, making it more "answerable" could be an option.
ie) Excluding anomolies (Movie stars/cross-over fighters/olympians) what does an average 10-0 GBP, boxer make per fight in a non-TV bout in Southern California?
In order for the obtained average to carry any substantial weight, the question needs to be a lot more specific IMO.
Television exposure skews the numbers.
Location skews the numbers.
Your promoter can skew the numbers.
Noteriety skews the numbers.
What I'm trying to say is this: Don't get caught up in the semantics of language and try to have a good conversation as opposed to a confrontation.
I'm not very good at it, but it's a good practice generally.
Peace.