I just noticed Carbatec have a cheap(ish) powered dust mask for $149. This uses standard AA batteries and you could use rechargables to lower running costs. The details are in their monthly PDF at http://www.carbatec.com.au/images/PD...August2003.pdf
I know everyone has been waiting for the Triton Mask to be released and this would not be a patch on it but it might be helpful for someone a budget.