Rupert,
SSL has less to do with how hackable your site is and more to do with how easily someone else on the network/web can look at the data going back and forth. The two are *almost* exclusive of each other.
If you're not concerned with the actual data being compromised either by sniffing or alteration, then there is no reason to add the complexity and overhead of SSL. If you're concerned with how easy it would be for someone to hack your web server and cause problems, you should look closer at how you handle user input, especially when you're databases are in SQL.
You can look online to find books (lots of books) about hacking and how to test your app for common vulnerabilities if you really want to get serious about it. You'll also be able to learn why I said "almost exclusive," above. A good book will probably cost you between $50-$80.
Just my two bits.
Regards,
Flint