I have noticed a 1% conversion rate to the Buy button. This is normal and anything over 1% is icing on the cake sales. That is actually 1 out of 100 sales which we notice with some of the apps we help our clients develop as well as the ecommerce websites we build for businesses.
Our highest conversion rate has been 7.3% within the first week, but that was because the client had an established user base for their other apps and they advertised it prior to going live.
I hope that helps and can help you with further suggestions on increasing conversion rate.