Great article on costs of query plans
I’ve not seen anyone go into this details before in explaining costing of queries.
Whilst I would avoid getting hung up on the specifics of a cost as these will change from release to release. Its a very interesting read about how and why cost optimisation is done and then some quirky un documented stuff that gives you a little insight into possibly the first plan that would be considered by the query engine if there was no cost to anything.
http://sqlblog.com/blogs/paul_white/archive/2010/09/01/inside-the-optimizer-plan-costing.aspx