Nelson's, hogs, and blood pythons are available at most expos. Mountain kings are less widely bred and harder to find. You may see them, but it is more likely that you will have to buy them from a private breeder online. Normal nelson's should be $75 or less, albinos slightly more. Hogs are extremely variable in price due to the many morphs available. Normals can usually be had for $100 or less. Normal bloods usually range from $75-200. Mountain kings vary depending of species and quality. The least expensive are generally AZ, usually costing $75-200. As far as which to get, the colubrids you mentioned are all usually personable and easy to care for once they have been established. Make sure you purchase animals that are already feeding consistently, especially with the kings and hogs. Blood pythons are a different story. A blood python will grow to be fairly large and some individuals remain defensive. They are also messy, require more space, and a balance of humidity and ventilation. I love working with them, but know that they are a bigger commitment than ball pythons or other beginner snakes.