Before taking on a project, there is always a question: "How many people will use this?" So how do I measure that "people", at least inaccurate.
For something quick, dirty, and fairly cheap, try something like the following process:
@vituosi makes some great points, but I use an easier approach. Take your most realistic competitor, (similar product, pricing). Choose a company that has not been around 10 years! I usually choose the poorest performing company with the same pricing. Not a hobby, but a real company struggling to make it.
Then find out how many customers they have. Some of them will actually tell you if you call them and pretend you want to sign up, write an article, or you can do some digging (for example web hosts by how many sites on the IP block)
Last, take the number of clients, multiply it times their lowest price point, and then divide it by two!
That low number will give you your WORST CASE scenario. I always use a worst case to measure if a product is viable, and also to keep development within budget and time. This helps you look close at the comeptitors, and use them a gauge of your success.