Installing SSL for e.g. your web- or mailserver is easily done if you know how. However, the
certificate you use for your servers will in this case not be "signed" by any of the big certification authorities (CA's). Mail and web clients hold a list of "trusted" CA's, and if they come across a SSL certificate not signed by a CA on that list, show a warning requester the user has to explicitly acknowledge in order to use your SSL-protected service.
So, the $100 your webhoster mentioned could mean one, or both, of the things below:
- They charge you $100 for enabling SSL services for your web presence,
- they charge you $100 for providing you with a signed SSL certificate.
There are ways around both (like, getting yourself a vServer with root / Administrator privileges, install required software yourself, and get a SSL certificate signed by a "public" CA like CARoot), but they are not perfect. (Administering your own server can eat loads of your time and might not result in anything stable, useful, or even sane; and last time I looked none of the "public" CA's was actually recognized by any of the "big" webbrowsers.)
I hope this helps. If it's a one-time payment, the $100 offered by your webhoster isn't too bad an offer, actually. Getting a certificate signed by a big CA is something usually only open to enlisted businesses, and positively costs heaps of money.