Paid search can only capture people who are looking for an app.... which is usually less than 3%. In general 3% of your target audience is looking to buy what you are selling.

A better use of adwords would be to use the display network, running banner advertisements that promote your app ONLY on sites that lawyers visit for industry news. In that case there are no keywords, its what sites does your audience visit regularly. I'd be happy to explain this more on the phone.