It depends what kind of SSL certificate we are talking about? For simple websites/small e-commerce you can buy even such simple certs like COMODO PositiveSSL for £3.41/year or RapidSSL for £4.56/year. Of course I recommend something better.
Business certificates like GeoTrust True BusinessID Multi-Domain with EV cost £214.78/year. And corporate certificates cost way more, even thousands of pounds, like Symantec Wildcard SSL for £996.93/year or Secure Site Pro Wildcard for £2,562.15/year
If you need a good SSL and one you can afford, go for GeoTrust like GeoTrust True BusinessID for £36.25/year or GeoTrust True BusinessID with EV for £83.45/year.
Installation of SSL itself is very simple. I do understand that some guy value his work, but seriously... £600? For me it's a scum. Today, almost all hosting providers offer possibility of self-installation of the SSL certificates. Even if you pay them to do the job for you, it won't be ridiculous £600.