Sorry for the delay. Been trying to meet a deadline. I went back and looked at the two companies I mentioned earlier for pricing. With those in mind, I'd suggest you charge $2-3 / mo. However, I also see that both of those sites are on a freemium model. So, there's a little more to think about. Would you charge for all levels of access, etc?
Before I discuss how I feel about price, I want to say that there is a market for it. People want to manage their to-do lists and tasks. "to do list" is searched for about 90k times a month on Google, and so is "task manager." Also, I've taken a little time to look at some of the competitors and here's what I've found: http://rivalseek.com/trends/index.php?market=4 Not sure what happened in June 2013 but the overall 3-year trend seem to be a 25% growth. And the chart doesn't really show this, but that's a lot of traffic. Todoist.com alone receives an estimated 12.3 million visits a month. From looking at different companies I also really liked taasky.com because they appealed to the fact that I wanted the system to be simple. But, still, I would not use them because I want a system that works on both my phone and my pc.
Personally, I'm still on the fence about which one I would choose. It's between yours and wunderlist. I like the simplicity of yours and the fact that I see it works wonderfully on both my pc and phone. My concerns right now are: a) will I need the additional features of wunderlist, b) am I going to pay for something I could get for free, c) am I going to be the only person paying for it? I'm saying these things not to discourage you, but to try to provide feedback and help you with sales process/objections.