Predicting the potential market size and product lifecycle?

[AudiSport]

Hi,

What data or resources do you all use to predict the size of a market you are looking to launch a product into? I'm aware of GFK, however the associated costs for such a subscription are in the thousands per month and not something we can justify. Additionally, it can be just as challenging trying to predict the life-cycle of a product and the levels of demand increasing, and then dropping off. I have not doubt this is very dependent on channel, category and the overall industry, however I'm sure some of the principles apply across all products.

Thank you for any input in advance!

[BGARK]

Specific product for what sector exactly? Is there a trade magazine if so they usually have subscriber numbers and databases for sale.. my 2p