The price per page is shown by adding together the price per sheet of paper (default value is $0.005, 1/2 cent) plus the cost of the ink/toner that you use to print the page (default values are $0.035 for B&W, $0.095 for color). The ink costs are doubled for double-sided print jobs since you print twice as much ink per sheet of paper, though the overall print job when in double-sided (Duplex) mode ends up costing less because of the lower number of sheets of paper.
So, if you want your color laser to reflect that the cost of printing one page to it is $0.35, you would set the costs in the GreenPrint Settings tool for your printer to $0.005 for the paper and $0.345 for the ink/toner. If the cost that you wish to show is more than a dollar per page, make sure the values you set reflect that.
It somewhat sounds like your currency is not the dollar, though. If this is true, you can also set this within the GreenPrint Settings tool - so that the currency symbol is shown properly. For example, the cost of a printed page in Rupees has a much higher numerical value - say 40 per sheet. You can set the cost per sheet for this too, as you wish.
The GreenPrint Settings tool is found at Windows Start menu->All Programs->GreenPrint->Tools-> GreenPrint Settings
These cost settings are found on the 'Printers' tab. The currency choices are found on the 'General' tab.