18% was reasonable for BCC back when it was downloadable. I got approximately 1.5% of downloads to convert, so that would be about .27% as you are measuring it. Different markets, different acquisition strategies, and different apps will have
vastly different numbers here. It is also highly sensitive to time of the year in my market.
If you haven't looked at it before, optimizing a) the installation, b) the first run experience, and c) the checkout funnel (hint: start that funnel in the application, ideally roadblocking a customer from something they urgently want to do) produce big wins for comparatively little investment. For example, for ~3 hours teaching people how to install software, you can get a big win in successful installations. (You are likely counting 18% of visitors initiate downloads but, unless you take efforts otherwise, you have no clue how many downloads actually result in successful installations or executions of the product.) Try downloading Skype and steal everything they do.
You can really increase download rates if you send targeted traffic at landing pages designed to encourage installation. (AdWords converted upwards of 25% for me, though I eventually "bid that down" by getting less qualified customers, because it was still profitable.)