That quite interesting. Personally I think that a lot of businesses aren't aware of how much of a problem hackers can cause so a lot of them are oblivious to it. Firstly you meed to make them aware that they need such a service before you can market to them otherwise they just wont buy because they don't think they need it. My advice is charge per fault found to get started with and if you're good word will get around and when you have more jobs than time, thats when you can charge per hour
Hope this helps and good luck
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