The question is important. Preliminary information I've gathered suggests 2-4% is common.
When you have a statistic in this range, it might seem discouraging. I prefer to look at it the other way: low numbers are much easier to double, so the opportunity for meaningful improvement is high. Going from 2% to 3% is a 50% increase in store traffic.
There is no guarantee that increased traffic will result in increased sales, but if it doesn't, that's valuable information. It suggests what you're selling doesn't appeal to your traffic.
There are different tools in Google Analytics you can use to track store visits:
Find it under Content > Site Overlay. It places numbers over all the links on your site page showing the percent of visitors who clicked on them. (If you are using Safari and it won't "turn off," delete your Google cookies. I recommend using a different browser for this reason.)
Find it under Content > Top Content. It's a table listing your most popular pages. Often, a single snapshot doesn't tell as much as tracking your store's position on the table every month, or after making site changes.
Use Goals > Goals Overview > Set Up Goals and Funnels. It's easy to set up goals. The hard part is remembering to check them later. A simple goal might be to measure how many people arrived on your landing page and eventually reached your store, regardless of the route they took. Any site with a store will probably want to set up some store-related goals.
You might be tempted to use actual sales as an indicator of store success, but I think they tell you about how your store is doing in its current configuration, not its optimum configuration. Keep in mind that if you have a successful design and you double its sales, it frees you from creating a new design with all its associated overhead.
Maximizing sales of each design requires maximizing the number of prospects who are exposed to it.
The store is a destination, so many of the methods for optimizing store performance involve routing traffic to the store, then making the visit a positive one once they arrive.
Someone emphasizing reader usability will probably say that your store should be called STORE in your navigation. Someone advocating the "koolstüff"* approach might argue for an eye-catching graphic, prominently placed, with a store name that is drawn from the themes in your comic. Using both is worth considering, possibly placing the koolstüff graphic near the end of the day's comic episode.
*"Koolstüff" is a term I made up in hopes of simplifying a wordy concept. It's all about taking the qualities that make your comic cool and appealing and spreading them out over other parts of your site to enhance its appeal to your readers. You can follow the link to the original article.