you've got to look at what it would cost someone to build it up compared to how much a new complete bike would be. I'm afraid that, if you set a complete build price of £400, then the frame allowance would only be about £50 max. Even going cheapo components you have:-
Wheelset - £100
Groupset - £150
Money left for frame, forks, bars, stem, seatpost, saddle, pedals etc - £150