It's cheaper because they only have to focus on 4C4 (4 choose 4) dynamic dialogue options at a time, not 8C4 or, like BG 1 and 2, 18C6.
How would that work exactly? You still can choose any combination of characters, so they need to be create anyway. BG1&2 companion content is rather basic. Most RPG have more complex and time consuming companions then BG1 or 2.
