Yeah, Tristan's bike has had about 8-9k spent on it.
Its not neccessary if your starting out.
If you budget around $3k for a new bike or as grnman said about $1500 to 2k for a awesome secondhand bike, something like a klx frame and 155z or yx160 in it, you wil be flying round the track and can build on that base pretty easy.
Thats what I did, I bought a bike for $1700 with the best of everything on it and will work from there. Just needs an upgraded engine rather than and upgraded eveything.
But if you just want to give it a go and see what its like, just get a set of cheep tyres or ask to borrow some and just use any old bike, just as long as your having fun!!!