Video games often follow a predictable model of sales. Several factors must be taken into place before making any such model.
In the United States, NPD December accounts for nearly 25%, while October, November, and December account for 50% of the calendar year's revenue. Additionally, history shows that from January to October within any given year the data for a system's installed base is roughly a linear function. This is always subject to change if there is a price cut or a system seller game like Halo. In addition, November's hardware numbers are often 2.5 times greater than October due to holiday's gift-giving nature.
The NPD top 10 can account for 25-45% of the market (in dollars or units). The dropoff from #10 to #20 for a non-holiday month is around 60K to 100K. See for example, Feb 08 and April 08. The console specific top 10 (e.g. Top 10 PS3 games) tails out around 30-60,000 units. At least 33k is needed to reach the Top 100 on November 2008, a holiday month. Despite not regularly reaching the NPD top 10, Nintendo DS sells the most DS than any other system.
The grey market can be used to gauge supply issues. For certain cases, there is a strong correlation in monthly unit sales and grey market prices from auction sites like EBay.