I did say very roughly. From eBay a 1500mm ballscrew with the bearing mounts is £115 but he's using a rotating nut so the bearings will be more expensive - call it £160. One additional 3Nm motor and 80V driver is £75 max. Better PSU needed, another £15 to supply the extra motor. Another two pulleys and a belt, £15. That's 160+75+15+15=£265 so I think very roughly £250 was a reasonable statement as clearly I wasn't including labour.
I might not pay revenue but I do pay VAT.
Who said anything about charging a high price? Imo the prices charged in the CNC industry are usually extortionate. I try to lead by example by charging around £6-£7 per hour for time. My objective is education, interest and not making a loss, not looking after other people's profits.
Why not ask them how much to add a second ballscrew? They might say it's too much work to change the design, but it can't hurt to ask.
Also in the listing there is far less information than there should be. It doesn't look like any of the pictures have the actual machine that listing is selling - the AR1200. Some of the bigger ones have vacumm bed and other nice things, you can bet that costs more.