Hi one of the best dirt jump forks around is the Manitou Circus Expert. Its a great fork and can be bought new within your budget. Heaps of people run this fork. It can be internally lowered too. Id recommend the 100mm version. This fork is so sick. it even has an air spring as well as coil and a knob with 8 settings to adjust stiffness. you could get a secondhand arge or 831 or even an older marzochi dirt jumper but. The circus is truly amazing. http://www.chainreactioncycles.com/nz/en/manitou-circus-expert-forks-20mm-2016/rp-prod117112