To even have a vague clue, you'd really need to be a little more specific about 500,000 tonnes of what kind of material?
If it's household waste than that's relatively simple, but if it's more dangerous materials then the costs would increase quite substantially.
Also, would the plant be purely an incinerator, or a heat-reclamation system, ie: collecting the heat from the burning and using it to heat water?
It's actually very difficult to find figures for even the purchase of such a unit as many would be bespoke, but I did find one that claimed a fairly big unit would be around $750,000 to purchase. This I'd assume is for biomass type materials (eg: household waste).
http://vietnamnews.vnanet.vn/2004-06/09/Storie s/26.htm
I'd imagine maintainance would be around $50,000/year and as for people, I can't imagine more than about 20 - 30 would be required even if the plant worked 24/7. Those are figured based on a best guess however due to the lack of information out there.