Hi Kev,
As far as I know, you can only go through Bunac to get the temporary work visa as they have a special agreement with the Canadian High Commission. So you kinda have to suck it up in regards to the £200. You get all the proper information though as Bunac sort all the visa application out for you, so all you have to do is fill out the forms. For flights you HAVE to get a return flight....this is the deal with the visa. Just so they know that you ARE going to return home after your visa is up. Insurance is also needed and Bunac require you to have this before they give you your work authorisation letter. The thing with both of these though is that you don't have to go through Bunac directly to get flights and insurance. If you did it would cost a lot more and there is no real benefit to paying the extra fees.
One bit of advice would be to not get really cheap insurance as if something does happen and you have to claim then you won't get much of a pay out, if at all. The group flights may be worth it if you want to meet some people in the same boat as you, if not you can always wait til you get to a hostel.
Anyhoo, this is all from what I have found out as I'm off to Toronto in september. Hope this helps!