Think about it this way, a ton of startups that get funded with huge amounts of money end up making $0/month :)
On the other hand, pricing is a real problem. I don't see many people paying $19.99 for it.
If you believe many people have this problem i'd suggest a very low monthly fee - although it will make less in the begining it could make more in the long run (as long as twitter doesn't implement a feature like this).
We just released a new beta desktop app that tracks what's going on your computer and than you can assign that time to projects and tasks at the end of the day. You can also use the traditional way to track time via a stopwatch.
We use the freemium model (Paymo time tracking & invoicing http://www.paymo.biz)
it can have the following benefits for your potential business (in no particular order):
1. helps spread the word (lower customer acquistion costs)
2. "free" users are more likely to give you feedback, which will make your app better over time
3. you and your team feel good that you're able to share with the world something for free (just think of how many free apps help you during the day)
4. you can convert free users to paying customers. (in our case the app actually helps the users grow their business)
5. people seem to be willing to try out free apps vs trial apps more often