Determining the number of sales at a given price point

I wish to develop a few jquery plugins and sell them on a site like http://codecanyon.net.

My question is, before developing a plugin, are there at least some guidelines as to estimating the possible number of sales I can get for a particular price? For example, tet us say I wish to develop a plugin that gives hourly updates on wind and weather conditions in the 5 great lakes. This is geared towards amateur sailors and watersport enthusiasts. Is there any way to estimate what kind of sales I can get for a price of $5 - $7? For a price of $2?

For those examples, I'd do some market comparisons. You can probably find plugins that are similar, or plugins and apps that seem to target a similar market. Google them online. Of course, no one will tell you how many sales they made. But look at the number of comments on the plugin page, or the number of downloads of the app. That gives you a sense of the market size. Find some public stories of people who shared their actual sales number, and correlate with the same parameters (number of comments, downloads, etc.). Plot a few of those statistics and try to figure out how they vary and see if you can estimate how your market will look like.

As mentioned above, you'll have to consider what your plugins do, whether or not other plugins do the same thing and if they do how well, etc. But codecanyon does tell you (a) the price to license a particular item and (b) how many people have bought that license. So I would suggest just looking there for similar products and looking at different price ranges.