This Amazon listing wants £295 for a ZX81
https://www.amazon.co.uk/Original-Sinclair-ZX81-Spectrum-Pro...
Or this TRS80 for £500 https://www.ebay.co.uk/itm/Radio-Shack-TRS-80-MODEL-100-PORT...
Of course they might not sell for the listed price, there are other listing with lower bids. I don’t know a good way to get an accurate figure for what things typically sell for on ebay etc.