CCTV System cost

Does anyone have a way to figure what people should spend on a CCTV system? I have been thinking of a way to determine what is a reasonable price, and have come up with a few ideas.
1. A percentage of the $ amount of the property/equipment being covered. I think if you have $100,000 worth of equipment sitting in a lot, $5000 would be a reasonable amount to spend to be sure you can monitor what happens to it. If you catch the person because you have footage it could pay for itself right there.
2. Percentage of sales (retail store or restaurant/casino). You know a certain % of your product is lost, what would it be worth to lower that amount?

These are just a few ideas, as they pertain to the types of business's I am targeting. I have had a lot of people tell me the reason they did not install a system in the past is because of the high price. I would like to be able to tell people either;
-Yes that bid was too high considering what you are trying to monitor, or
-That is a reasonable amount to spend on a system, and here is why it is worth it. (ex. 1 above...)
Unfortunately cctv is one of those items that will only pay for itself when negative things happen.

I am looking for feedback/comments so if you have any thoughts let me know.