My data are probably relevant to you.
Product: $20-$30 image processing niche Windows application. Almost 2 years old now.
Competitors: Two very similar $30 products and several free solutions like gimp and photoshop plugins. These are usually harder to use, therefore non-tech savvy users pay me instead.
Total website traffic: about 2500 unique IP addresses per month, 50-60% coming from google, 70-80% total from search engines.
Average visit to download ratio: 30%
Monthly download to sale ratio: between 7% and 12% - apparently seasonal
Google webmaster data for the last 30 days:
Total search impressions: 9800
Total clickthrough: 1200
Top 5 keywords add up to less than 800 impressions and 100 clickthroughs. 70 of these are the product name itself.
Total number of different search keywords: 350
My top 5 keywords have between 200 and 3000 monthly searches globally according to the google keyword tool. Pretty much all others are below the bar and cannot be measured.
The best part is that these niche long tail keywords have extremely low competition, therefore getting to the top 3 search result spots is very easy. Validation hint: if you see paid google ads appear for a keyword, there is probably profitable demand there. The more ads the more profit to be seen, plan your SEO accordingly.
I hope that helps.