I think the diaphragm cost may be closer to $50 but you also have to add in the cost of the spermicide jelly, which you can buy at the drugstore. How many tubes you need per year depends on your use!

http://www.americanpregnancy.org/pre...diaphragm.html
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How effective is a diaphragm?
The diaphragm has a failure rate of approximately 5% for correct and consistent use. Typical use, which is the average way diaphragms are used, has a failure rate of approximately 18 - 20%. This means that 18 to 20 people out of every 100 will become pregnant during the first year of use.

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Correct use means put it in every time and use the spermicide every time. Best of luck!