Kookaburra cost me a bit less than 30.

Drill battery-£free
2x servos-£10
Ally tread plate-£1.50 (scrapyard)
Spectrum rx-£6
Chopping board-£3.50
Wheels-£free (off a pushchair I found smashed up)
Bolts-£2

Total:£23

Of course this is just an example, but it can be done quite cheaply.