What you are describing is "rainwater harvesting'. It's not real practical in these parts because we get all of our rain in the winter, then nothing for 6-8 months. The basin would just fill up in Nov. and stay full through Apr./May then would go empty for the rest of the year. In climates with regular rain, folks will dig a very large basin and collect all of the runoff and use it for irrigation. The waterfeature keeps the water clean so it doesn't get stagnant. Kind of a cool idea.
Like was mentioned earlier, if you dump runoff into the water feature, it will overflow and need to be drained off. As far as pricing goes, your price sounds way too low. The materials for the job you describe will be close to the $1500 mark. A pump is ~$300, vault is the same, you need a falls which is a couple hundred and liner for close to a hundred. Add in your gravel, pipe and labor and you will need a lot more than that. Our pondless systems start at $5,000 and go up from there. Sometimes we will offer a special with 10% off, but that is a long ways from the $1500 you mentioned. Water features are a luxury item and if you can do them properly and make them beautiful you can get good margins on them so don't ruin the market where you are by giving them away and remember that there are always hidden costs you don't think of at first - not to mention tools you will need.