I believe the basic swb kit starts around 8-10k but theoretically all you need is a defender and some paint. You can build one for around 25k which sounds a lot until you realise the equivalent defender, galv chassis, fresh paint, cage, suspension, winch etc will probably cost you a lot more and you end up with a bespoke truck that is more capable and suprisingly practical (I daily mine) I was very lucky and found an unstarted kit for sale with quite a lot of bits missing. So although mine worked out cheaper I had a lot more work to do and a pile of bits I had no choice but to buy from foers (they are more than happy to supply parts for older ibex).
Mike