Something you should read which might help you know more about knowing what a good conversion rate is. Average cost per click is also something you should consider looking at:
If you really want to have an industry benchmark on Google conversion rates, give our AdWords Performance Grader a run. Otherwise, you should look at a metric called cost per lead. Take the average cost per click you are paying for an ad and divide it by your current conversion rate and you will arrive at cost per lead.
(e.g. If your ad’s CPC is $1.00 and the conversion rate for the visitors that come through that ad is 5%, then your cost per lead is $1/.05 or $20 per lead)