The return you get will vary massively depending on how your website converts, and yourself, pricing etc. as well as what campaigns your running, competition, how well it's set up/managed etc
Adwords can get people to your site...what happens after that is down to other factors.
You need to look at the bigger picture. Adwords job is to get people to your site (or a call if using a call only campaign) Your website then needs to convert into a phone call...then you need to convert the call into a sale...and don't forget about about lifetime value of that customer and even referrals.