I know Caster will be here shortly to comment.
I don't cast but I have read up on it. You could compare cast bullets to loaded cartridges. You can reload yourself and same a little money but it's more about being independent of the market which is unstable at best. Also you can make loads that can't be bought on the market. Same with cast bullets, you might save a little, but as long as you could get ahold of some lead somewhere, you could make bullets. There are some bullet styles that aren't even made if you want something specific. Heck there are bullet molds that aren't made anymore, such as some Lyman molds and others.
It's something I want to get into eventually simply for the knowledge and independence.
You can make blackpowder if it comes down to it, but I'm unaware of a way to make primers.
Edit: mike said basically the same thing as I was typing this, oh well.