I have no idea on cost...
the d20 system is a complets combat AND character development tool (skill points, etc.). As for plot development, honestly, I've never seen a "rules system" for that... I can imagine rolling a d100 on the "sub plots" table (shudder)... Really, if you want to give players help in that area, that can be written into the rules books in a Storytelling section, drtailing the basics of good plot, kinds of conflict, hero/villain relationships, etc. independent of the "rules". This is where knowlege of gaming history would help you- for instance, if you oened a set of the origonal White Wolf Vampire, Mage, Changling and Werewolf books (particularoy the Storyteller's Guides), they have pretty extensive sections on establishing a game Theme, setting mood, basics of good plot, etc.