The main problem I see is that filling in information sucks, and you have to fill in a lot of information to get an answer. The other problem is that people who bother to keep track of all that information probably already use something like Mint. Mint already has a Goals feature that let's you do something similar, and they already have all your data.

To get around this problem you could make a bunch of assumptions and just present people with some good financial tips in the form of questions.

For example, let's say the person says they want to buy a $10000 car.

Your app would respond:

Are you paying in cash? If so,

- Do you have enough saved up for an emergency? You should have at least 6 times your monthly expenses saved for emergencies.

Are you going to make monthly payments? A 3 year loan for $10,000 at 6% interest will cost you about $300 per month.

- A car should cost no more than 1/3 of your annual income, do you make at least $30000?

- Your monthly expenses shouldn't exceed your income, do you make at least an extra $300 per month?

- Some of your monthly income should go towards retirement. Are you saving at least 6% per month?

Etc.

Then you could offer people the option of putting in their information to make the feedback more accurate and personalized.