I think what you're saying boils down to:
"I'm a great web app developer. I'm not sure what to develop. One thing I can do is take an existing solution and deliver a viable alternative, at a lower price."
You're describing a classic market entry strategy - one that can be successful or blow up in your face.
Let's think about the main issues.
First, it's true for just about everything you can think of online that there's no bottom price buffer: what your competitor does for $20, what you'll do for $10, someone else will do for $5. If price is all you have long-term, it will never be enough. So, be realistic about the window of opportunity low price buys you.
Second, it's often the case that in competitive markers, prices move so that expected profits move to zero - that is, that acquisition costs are only barely matched by revenues. There's no universal formula, but crudely, monthly fees X average months to customer churn. The market leader may be showing you something about their experience of the market - and low pricing may be a trap. Part of Google's profit engine is all about harvesting that, increasing the market cost of acquisition through keyword bidding. So, be careful to budget promotional costs accordingly.
Third, you need to be very clear about your proposition and how it interacts with the established offering(s). Are you going to be essentially a clone, are you going to do more, are you going to do less, or are you going to do it differently? You may already be clear about this, or it may be a choice you face. You need to think hard about how to communicate value to your customers. For instance, if you decide to deliver 90% of the value for half the cost, you have a clear rationale for pricing in the mind of your customer. If you're up against a minimalist, but premium-priced product, you need to avoid a situation where potential customers read the price difference as suspicious, an indication of problems. So, copy products can't just be cheaper, they must always make sense of the price differential for the customer.
Copy products don't usually win kudos with investors, because by definition they tend to find niches in other people's spaces rather than create new markets. But in my view, if you're careful, realistic and concerned to give real value to customers, there are great opportunities in the world of "me too."
As Henry Ford said, "The man who will use his skill and constructive imagination to see how much he can give for a dollar, instead of how little he can give for a dollar, is bound to succeed."