Cost effective? No such thing when it comes to this lovely hobby

Jokes aside, its hard to get a cheap start, this is also true for
BA. The Battleforce is OK, but those 10
Tac should'a been something else
imo. Maybe a dread or another 5 RAS. However, if you dont mind converting:
Buy their battleforce and convert some
Tac marines to RAS using the
JP from
DC.
That way, you'll get 10 RAS, 5
DC, 5
Tac and a Rhino. And since you're
BA you should sell those
Tac to someone else to get some money back