California is a very large state, and there are lots of places you can find a home under 300k. Of course, this all depends on what type of home you are looking for. There is a big difference in prices between a tiny 2 bedroom starter home and a 5 bedroom large family home. Land also plays a factor, as a small home with lots of yard space or even acreage can cost more than a larger home with a small yard.
In general, you will want to stay away from San Francisco, Los Angeles, and San Diego, as these are the areas with the highest average home price.  The smaller the town, and the farther it is from these cities, the lower the price. The cheapest area when it comes to home prices is San Bernardino county. Many of the cities in this county are only an hour or two from Los Angeles, so you won't be too far from the amenities that a big city can provide. While the Inland Empire area of San Bernardino county has a reputation for high crime, if you look around there are still plenty of nice neighborhoods to be found. Loma Linda, Yucaipa, and Grand Terrace are all great places to start your search.