1. Go out and tell your customers "I have this product, it does X, Y, & Z" (make sure you use the present tense).

2. Tell them it costs $D

3. Observe

4. Change D as necessary

5. Repeat until the reaction you consistently get is "Wow! That's awesome, what's your website, I'm going to buy it when I get home."

Market research is the best way to figure prices out. Alternatively, as yubrew said, comparables works. Think outside the box when you're trying to come up with indirect competitors. We are building a subscription-based web app, but an alternative to our service could be a book. That books costs a certain amount of money. Thus, we can charge at least that amount, plus a premium because, well, our service is a lot better.