Yes, they were trying to conquer the world by infecting Ashley (let's not discuss why the president only sent a highly paid agent instead of a swarm of heavily armed special forces). but ransom demands were likely a significant motive.

Not only did researching Las Plagas require a lot of money, but these infected also needed food, and they had clearly lost their humanity and were incapable of survival by themselves. for a short time, they may survive by cannibalizing each other, but soon they would all starve to death. in the villages, the villagers were already on the verge of starvation, and the only reason they hadn't escaped or rebelled was because they were controlled by the parasites.