That is normal. A small amount of money buys a large stake in the early days. As the technology gets closer and closer to reality the amount it costs to buy goes up.
However don't read the above as invest in the early days - there are a lot of seemingly great ideas that will turn out to fail for some reason. Honda's bet is much less risky because it seems much more likely that cruise will have something to sell in the future. GM's bet was more risky, and [I forget who they bought cruise from] made an even more risky bet. This time it looks like it will pay off, but not all do.
However don't read the above as invest in the early days - there are a lot of seemingly great ideas that will turn out to fail for some reason. Honda's bet is much less risky because it seems much more likely that cruise will have something to sell in the future. GM's bet was more risky, and [I forget who they bought cruise from] made an even more risky bet. This time it looks like it will pay off, but not all do.