Hmm.
I'm seeing Shimano DH-2N30-E 6V 2.4W dynamo hubs on ebay for $35 (free shipping), and Foxeye StVZO 40 Lux dynamo headlamps on ebay for $13 (free shipping).
$35 + $13 = $48. Dynamo hub energized headlamp system.
You'd still need spokes and a rim - but a bike needs those anyway.
Hmm.
That sure would suit a dedicated errand bike. Even if I decided to sell it later that would make a good selling feature.