Tripwire company really cares of customers and after you paid once for their product all updates (new maps, weapons, vehicles etc), patches, fixes, features are for free. Both TWI games - RO:O and Killing Floor - got great stuff after release.
They already announced two new tanks and infantry transporters which will be released close after final release. It's obvious we will get new maps too.
Only paid, unharm and cheap addon, DLC were cool skins for Killing Floor game.
And what's also worth to noticed, RO:O and Killing Floor games cost same like DLC for Other games...
ps. Also there will be a lot of free mods, maps and skin packs from modders and fans.