2. Check your competition to see what they're charging (counterpoint: http://en.wikipedia.org/wiki/Blue_Ocean_Strategy)
3. Pick a number (ref: http://network.businessofsoftware.org/video/dharmesh-shah-on... - worth watching in its entirety)
4. Play with specials - this week, we're offering a 30% discount to all new users (better than decreasing the price outright and incurring the wrath of existing users)
5. Model the numbers, at all times, how many licenses do you need at x price point to break even, etc.
Many people make the mistake of underpricing because of this.
I don't mean to step on any toes, but I've heard "$20 is sort of pricey for a business -- why not make it $10?" enough to be skeptical of the advice on pricing from other people, unless they are saying Charge More.
That people are often willing to pay far more than you'd think is certainly not limited to enterprises and the wealthy.
You really can't figure out a price point in a scientific way (sample and control)... you just have to adjust it organically and see.
How is charging different amounts to different people "dangerous legal territory"? Companies do this all the time...think about all the transactions where the price is haggled back and forth, or companies offering a special discount to startups or nonprofits, or offering specials on certain days of the week, etc.
Unless you're segmenting your groups by some protected status, like gender or religion, I don't see how it's a legal matter at all. But mostly I'd like to hear why you know that it is.