To justify an investment you need to be able to come up with at least some rough estimates of the returns, or at least if that's a bit sobering you want an idea of the best case scenario (this will likely include the product actually getting made.). So I look at indie games targetting the same niche and say that if everything goes well then I'll sell as much as they did, but how do you go about getting this data? Do any online portals like Steam publish sales figures? From googling I seem to find a lot of press about sales rankings, e.g top 10 games in July and so on, but this is not much help.

One of the major problems with our industry is the lack of market data. The film industry release viewing/revenue figures as standard, which helps producers who are looking to sell distribution rights for a film or raise other forms of investment to get a picture made.

The games industry doesn't have this. The only publicly released figures are the headline figures "Mario sold XXXX zillion in it's lifetime" but with nothing to compare that too its useless for the purpose of projections. A few companies like NPD Market Research and Chart track provide some figures but they are pretty expensive.

You can find some numbers at vgchartz.com. They are less accurate than NPD, but they are close enough for trends and estimates. They track their numbers differently than the other groups so it is interesting to look at for comparison purposes.

This may be good enough for what you are searching for.

I'll take a look at those websites thanks.