This is a difficult question to ask because you can lessen your CPC which will do two things: 1: Lower your cost 2: Decrease conversion rates (I almost guarantee this)
What you may want to do is figure out how to make your current set of keywords (at the current cost) convert more often. I would start with A/B testing your ad copy and A/B testing landing pages. Once you get a handle on that, move to multi-variate testing. I'm sure there are some programs out there that will automate the process for you... but I just use excel to mass produce ad copy to cover every permutation of an ad, turn them all active, delete the under performers then start over again.