I am understanding you are mean the whole hive, not just the colony (defined as 'a group of animals living together'). Beehouse owners may seriously skew your results as these are likely to be single hive owners. You may be able to account for this, but that is another variable which may be affected by new beeks and ones with several hand-me-down WBCs.
You are asking for the average value which includes nucs.
I would poll for the value of nucs, mine are £125.
I would poll separately for the value of colonies and I would call a colony, for this purpose, a floor, brood box, Crown board, roof, and 11 frames with bees and queen with at least 8 frames of brood. Spring value.
If I have four nucs at £125 and ten colonies at £300. I am reducing the value of the colonies. That average is £3500 devided by 14 =£250
Yes, you are right, two polls might have been better, but I didn't want to stretch peoples patience too much . On the other hand, you are increasing the value of the nucs. I think people with more than 5 colonies are liable to be running nucs in about the proportion you suggest. It should be possible to capture that by inference and correct for it.