Recently I sent my brother (who dabbles in SEO) a email about some numbers I was thinking about relating to my CBX.GridMapping project for Unity. I was trying to come up with very small very reasonable numbers that I thought would be possible to achieve. Below is a copy of how the conversation went.

I sent my brother this first email ...

The goal: To sell 2500 copies of CBX.GridMapping tools at $10 over the course of 5 years earning me an average $291 month in income.

http://unity3d.com/company/public-relations/

1,000,000+ registered users as of april 2012
0.25% of the 1,000,000 users = 2500 users
2500 users times $10 = $25,000 in gross sales earnings
minus unity's take of 30% = $7,500 asset stores take
$25,000 - $7,500 = $17,500 in net earings

$17,500 by 60 months (5 year time span) = $291 a month

Given unity's rate growth and expansion into new markets this seems like an achieveable goal. The question is wether 0.25% of the registered
unity users are interested in my tool and if they are willing to spend $10 for it. $10 price point makes it the cheapest on the asset store
for paid tile/grid mapping tools. Other similar tools cost $25 to $50 & CBX.GridMapping will eventually contain a similar feature set. 

==== He responded with this email ... ====

You made a common mistake, based upon your calculations your assuming one million people are going to view your product (or 100% of unity members) and 0.25% or 25,000 of them are going to buy your product, you can aim for a 1% conversion rate on sales but if unity only has one million registered members your looking at the following calculation.

1,000,000 unity members
2% or 20,000 members see your product
1% or 200 members purchase your product
$10 x 200 = $2,000 sales
$2,000 - 30% unity take = $600
$600 unity take - $2,000 in sales = $1,400

If this is a five year goal your looking at $23.33/month over five years not $291/mo

Now a 2% view-count on your product with no advertising may even be a touch high, however with some advertising and promotion over the course of a few month's you can increase the view-count to more than 2%, in order to reach your goal at a 1% conversion rate you'll need a view-count of at least 20 - 25 percent.

But even more you have to take into account what is the approx unity population buying this specific type of product. For example if unity has 1 million registered shoppers and only 20,000 of them shop for this specific product your sales numbers will drop even more, even my calculations above leading to the $23.33/mo figure is assuming all 1 million members look at or purchase this type of product and that simple is not true.

This is probably why the others price there products at a higher price point because they know only a fraction of the total unity members are going to buy it.

So bottom line you need to think about it slightly differently, basically the break-down is this (For selling any product)

#1 - Determine size of market (Unity = 1,000,000 members)
#2 - Estimate what size of that market are interested in your product (In your case un-known but a slightly accurate estimation can be derived if the asset store show's how many sales a particular product has or a particular user has)
#3 - Of the estimated size of the market interested in your product, determine what percentage you can get to view your product (Click-Through-Rate [http://en.wikipedia.org/wiki/Click-through_rate] can be increased via advertising, ect.. no advertising means little visability)
#4 - Of the people who see your product how many will buy your product (Conversion Rate [http://en.wikipedia.org/wiki/Conversion_rate] - For amazing product can be as high as 5 or 10% for average product it sits around 2% for product from unknown brand sit's more around 1%)
#5 - Now that you know the percentage of users who buy your product after seeing it (#4 conversion rate), you can calculate how many estimated sales your expected to achieve.

Remember those five points and apply them to every product you'll ever sell in order to get an accurate number on the income you can achieve, the more information the more accurate your numbers become.

==== And my response to that was ... ====

your math is depressing    lulz :p