1. Apocalpyse: Core Rules $19.99
2. Starter Set: Depends on faction
Forsaken Starter Warband: $39.99
Brood Starter Warband: $39.99
Skarred Starter Warband: $49.99
Outcast Starter Warband: $59.99
Dragyri Starter Warband: $59.99
Core Starter Warband; $79.99
3. Other miniatures to get up to 500pts.
Models range from $9.99 on up to some huge models, so it greatly depends on what you are getting. On average though, a couple packs of 2 models around $19.99 apiece can be done.
Overall you can get into Dark Age for less than $100 dollars out of pocket.
While there are 3 hardback books to purchase, you only have to buy the the core ruleset, the rest (without the fluff) can be downloaded on the Dark Age site. Downloads on the site include special rules and weapon abilities needed to play, the stat cards, and templates and markers needed to play the game.
For more information, the Dark Age website has more pricing etc.