There are studies showing that individuals are increasingly using mobile devices to browse the web. This is especially true if your website has a blog that you frequently update—I tend to read blogs and other daily news sources on my phone.
Do you know what your site looks like on a mobile device?
How are people viewing your site?
Google Analytics easily allows you to find out how people are viewing your website. If a decent percentage of visitors are viewing your site on a mobile device, creating a mobile site might be worth the price.
Use a mobile emulator to simulate the mobile experience
Google Analytics will tell you how many people with iPhones visited your site on a given day and what pages they viewed. There are at least two sites that allow you to simulate what your site looks like on an iPhone, TestiPhone.com and iPhoney.
You could argue that the iPhone version of your site is more important than the BlackBerry version, given that people using iPhones are much more likely to surf the web.
Another site, dotMobi, can emulate what your site looks like on other phones (although it only has two options). I do not use a BlackBerry, but this emulator looks fairly close to the BlackBerry mobile browsing experience.