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Take off the VAT, £250.
Bike builders earn £7.75-£8.20 per hour, call it £8.00/h, add on the Employer's NI contribution and you get £8.45/h. So you can afford 29.5 person-hours before making a loss, not including consumables and static costs (utilities etc).
Halfords expect bike builders to do 4 bikes per hour regardless of type, which says a lot about their quality expectations.
Cost and price are two very different things. If they build it, they also need to add a bit to pay for dealing with any faults caused by the service rather than the parts or user.
£300 difference for the RTD80. Is it really £300 to build a bike?