The high priced seeds tend to have more stable genetics. Also, people can charge $300 because they've built a reputation for the seeds such as being Cannabis Cup winners.
I actually asked this question to a seed breeder I met at one of the Cannabis Cups and he explained that with a lot of the budget seeds, people are actually buying the high priced seeds, seeding out the plants and reselling the seeds so instead of a F1 hybrid you could be getting a F2 hybrid, for example, so your plants will not have the consistency.
In many cases, however, you can get high quality from low costs seeds but ultimately you get what you pay for. I was able to get some outstanding Durban Poison x Northern Lights seeds that were being given out for free at the 2005 Cannabis Cup.
The high priced seeds don't necessarily have higher yield but there are other qualities that are important such as taste and aroma. As an analogy, people don't buy fine wines based on a higher alcohol content.