The fog is clearing on booking and upgrading

Thanks to all the really helpful posts on here, I'm starting to find my way through the misty maze of booking and upgrading but I'm still a bit lost on booking a low code and then paying to move it up for a miles upgrade.
Am I paying for K and then only paying to upgrade it to W or S for a G on the basis that a G might not appear? Or am I doing it that way because you all know that it works out cheaper than buying W/S to start with? Do you sometimes find that it would have been cheaper to buy Z to start with?
I'm sure you'll all sigh at this but it is so useful for a newbie like me to get information from you frequent flyers who seem to know every aspect of such a foggy maze.
Thanks.
Whiterose
Am I paying for K and then only paying to upgrade it to W or S for a G on the basis that a G might not appear? Or am I doing it that way because you all know that it works out cheaper than buying W/S to start with? Do you sometimes find that it would have been cheaper to buy Z to start with?
I'm sure you'll all sigh at this but it is so useful for a newbie like me to get information from you frequent flyers who seem to know every aspect of such a foggy maze.
Thanks.
Whiterose