CPM ads are little cheaper than cpc adwords ad.
You could use CPM(display ad of google) to create brand awareness. However, if you need sales than I would suggest you to bid on good traffic keywords (this might also not guarantee 100% sale). Only few of them would convert. But users will come to know about your brand by actually landing on your site.
Keep on investing if you have bucks and meanwhile focus on seo which can give you free traffic.