That's only for 3 domains, not 100. 3 domains at $30/year is $10/year/domain which is no different than buying individual certs.
The multidomain cert supports up to 100 domains, but the cost is $29.88/year for the first 3 included, plus an additional $12.88/year for each additional domain.
Under this price structure, you could have 100 domains covered with one certificate, but it would cost you $1,279.24 per year for that single certificate.
3 domains at $30/year is $10/year/domain which is no different than buying individual certs.
The problem was that shared hosting plans didn't support multiple certs, forcing people with a few sites to purchase a plan for each. The multidomain cert solves this problem.
"Base certificate costs $165.00 for three domains"
"After the third domain, each additional domain costs just $45.00"
http://www.positivessl.com/multi_domain_ssl_certificate.php