Short answer: You can
Long Answer: When you "purchase" a SSL certificate, you really purchase someones insistence (not a guarantee) that you are who you say you are. Verisign is a well know company that sells this kind of insistence in varying levels of confidence. When you purchase the greatest insistence level you will have a background and financial check made of your entity.
In addition, nationally recognized certificate issuers are also natively recognized by prominent web browsers and will not present the web user with a warning that the issuer is unknown (web users can always add your certificate to their browser cache, but many are afraid to do that for security reasons).
If your site is not going to pass financial data back and forth then you probably can use your own certificate and your users put up with the warning inconveniences or use the shared certificate supplied by many web hosts - usually at no charge. Otherwise, get an appropriately signed certificate.
Also, there are many recognized certificate signers besides Verizign that are also cheaper. godaddy.com supplies many of ours for $19.95/year.