I am the designer/creator in which I posted the video and Instructable. I am really enthused about all the interest and the fact it has been embedded here!

The costs roughly as I vaguely remember:
Batteries: £40 each
Motors: £25 each
Speed control: £80
Wiring: £20
10″ rear wheels: £10 each
Various bearings bolts and fixings: £20
Box section mild steel frame- found lengths of it at school
BMX front end I already had

Here is the Instructable:

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<font size=”1″>600W Electric Trike – More DIY How To Projects</font>