Your vendor charge seems right. A SSL cert requires dedicated IP for your site, and $24/year cost is correct. Even you can find out some cheaper offers (might be $12/year), but as you mentioned that you are very happy with your host, you should stay with them. The difference is not much ($1/month) enough for your risk to move to a new host.
Can I ask $15 is for installation only? Or it's for one year of SSL cert already?