I had to write a business plan once, and making sales forecasts for games was the most annoying part of it.
In the film industry, box office sales are figures made available to the public, in the case of almost any movie you'd be able to see at the cinemas.
In the music industry, sales can be determined by the amount of blow found on an artists groupies
In games however sales figures for successful or unsuccessful titles are rarely every disclosed/published and even if they where it wouldn't mean anything.
Since your game could be similar to another in almost every way and yet it wouldn't necessarily mean it will sell anything like that.
In the end, after some degree of resignation, I resorted to doing all that I could, hunt down what figures you can, even really vague sales data like "has sold over 10 million copies", quote those figures, make up unfounded yet "realistic" estimates of your own sales and then attempt to somehow justify the connection between that estimate and your vague collection of industry figures.
Hope that helps