It can vary widely between different industries. What is a high conversion rate in one vertical can be extremely low in another and vice versa. As have been mentioned below, it also depends greatly on the type of search. For branded search you would normally expect to have a much higher rate than for more generic search terms.
Another factor to consider is how compelling the results surrounding yours are. A site could be ranking below yours but have a stronger call to action and be performing better. You should think of your page title and META description in the same way that you would the copy for an Adwords ad.
Here is a useful article from Search Engine Land on the topic of optimising your click through rate for various different verticals
I would also considering checking out the book Cashvertising which has hundreds of quick and easily digestible tips on writing strong copy.
Another major consideration in recent SERPs is the presence of answer boxes which can greatly improve your visibility and potentially clickthrough rate. You can find a guide to how to get into Answer boxes in the SERPS here
Also if you are not using schema for elements of your content such as reviews you should consider this. The star ratings that you see in the results if you do searches such as seo companies uk are generated by use of schema and can have a significant impact.