To answer that question would depend somewhat on what type of king snake or milk snake you are interested in. Corn snakes (Pantherophis Guttatus) make excellent pets, feed well, and are generally very easy to handle. I have noticed Okeetee corns do tend to be a little more nippy then other forms of corn snakes but as they mature this seems to sub-side. This snake is generally considered the first domesticated snake and is available in a wide variety of designer morphs.
Milk snakes are gorgeous and some of the larger ones will tolerate handling better the some of the smaller species. If you are seriously interested in a milkshake I would recommend a Honduran (Lampropeltis t. honderensis) or a Mexican milk snake (Lampropeltis t. annulata). Both of these species are relatively large for milk snakes reaching 40"-50". Both also tend to mellow out more as adults and become more tolerant of handling. That is not to say that other milks will not tolerate handling but experience has taught me they are far more nervous when handled though I have maintained other species of milk snake in which individual animals calmed down where as the species in general was considered nervous. In snakes as with people each individual will have characteristics specific to their personality.
King snakes are another topic all together. Their temperments and care requirements can be quite varied depending on the subspecies. I would stay away from Grey-banded king snakes (Lampropeltis alterna) and tri-colored kingsnakes as a beginner. These snakes can have feeding requirements and care requirements above the skill level of a beginner. Most of the king snake species I am going to recommend will stay around 40"-48". They are all cannibalistic (milk snakes have the potential to do this but it is by far not as common) and should not be housed together. All of these king snakes from my experience are extremely voracious feeders which should not be confused with aggression. They are all reasonably tractable temperament wise and all make good pets that generally tolerate handling well. They are as follows:
California King snake (L.g. californiae)
Flordia Kingsnake (L.g. floridana)
Eastern or Chain King snake (L.g. getulus)
Speckled Kinsnake (L.g. holbrooki)
Desert Kingsnake (L.g. spledida)
All of the animals I have listed are fairly common on dealer and breeder lists. They may not be as common in pet stores. Price will be somewhat dependent on the animal you pick. Some are more common then others and some of the morphs available are more common then others. In general you will find prices ranging from $15-$2500 and a few morphs that may be even more expensive. In there nominate forms none of these animals will run above $150.00