There are numerous reasons for this but I'll list a few:
1. BlackBerry told us app size should not be more than 2 megabytes. BlackBerry phones do not have a lot of storage for apps so they need to be smaller. We put as many sounds as we could into a size of around 2 megabytes. The iPhone version is almost 40 megabytes and would take up too much space on BlackBerry phones.
2. BlackBerry has a minimum price of $2.99 (or free). We are charging the minimum price.
3. BlackBerry costs us more in terms of development and support. The large number of different phones, different screen resolutions, etc all have to be tested. The iPhone and iPod touch are basically the same device and have same resolutions so testing is much simpler.
4. The demand for apps is much higher on iPhone than any other device which in turns means we can lower prices. iPhone and iPod touches have a large amount of storage (>= 4gigs) so size is not a factor.
Hope that helps to understand the logic behind the price difference. You'll find this to be the case for many popular game titles as well. Cheers, Todd