What are the differences in care between males and females?
There is very little difference in the care of females vs males until you reach a point where you intend to breed them or not. Up to that point care is essentially the smae for both sexes.
Do males get larger, and as such need bigger enclosures?
Length wise there is little if any difference in size. The Females will have a tendency as they mature to be slightly thicker and heavier bodied. Females may attain a slightly longer length by a few inches at max size but this hardly rates a differnece in the size of the enclosure needed to house them.
What do you think is a reasonable price?
Thats like asking someone how long is a piece of string without letting them see it. The "typical" commonly available color morphs should retail for 29.99 to 39.99 more times than not. Going through a breeder the prices can vary much more so depending on the color morphs availablity and the number of snakes available from the breeder. Some of the more striking color morphs can command a considerably higher price on the market.
It seems to me they can live in a 20gL for life. Is this true?
I beg to differ but many others will disagree. A Neonate will be quiet content for some time in a 10 gallon tank or a 20L. As your snake gets longer it should be housed in a enclosure thats at long as the snake minimum. Even my larger Boas, as sedate as they were, were kept in enclosures that were 8ft (L) x 3ft(W) x 5ft(H). Your snake will need some room to move around, flex its muscles and exercise a bit from time to time. I think for an adult Corn a 55 gallon or 75 gallon enclosure would be reasonable for 95% of the cases.
As far as other question you have, those could probably be best answered by doing a internet search on care and care sheets. Realize that some of these will vary from author to author, so look at several sources and come to a consensus. Do use a length of enclosure that will allow you to have a area thats considerably warmer and a end thats cooler with room in between so your snake can find a happy medium that suits them the best. DO NOT use heat rocks that are commonly sold in the reptile section of the pet store. If your snake is always on the hot end and rarely moving or the cold end then you need to play around with the temerature of the enclosure. A snake thats always on the hot end is a sign of a snake thats in a enclosure thats too cold and vise versa.
Have a way to increase the humidity in the enclosure especially when your snake glasses over as it prepares to shed. This will help your snake have a clean and complete shed of its skin. There are a number of ways to do this, so shop around and consider this carefully. You also need to have a water dish thats large enough for your snake to soak in during this time as well if it needs to.
Feed your snake in a different contianer than you house it in. This will prevent your snake from becoming accustom to the learned behavior of "everytime the cage is opened it must be dinner time". This will help you keep your snake docile instead of being a "belt fed staple gun". This will make cleaning the cage and doing day to day maintenance a lot more pleasant for you and your snake. It will also make handling your snake a easier thing to do, which is a good thing since there is a good chance you will need to take it to the Vet sooner or later. Finding a good Vet that speacializes in snakes is tough enough as it is, they often have two sets of care prices when you do find them...one price for "nice" snakes and one price for "Not so nice" snakes, some will flat out refuse to treat a snake that cant be controlled or restrained. This shouldnt be too much of an issue with a 4-5ft Corn as opposed to a large heavy bodied 12ft Guyana Redtail Boa! Something such as a 12 gallon plastic tote should suffice for a Corn at feeding time.
For feeding, get your snake trained over to freshly killed or thawed previously frozen prey items. A live prey in your snakes feeding cage can cause your snake serious injury potentially if not closely monitored. Even when they do strike and wrap up a prey, it can take a few minutes before the prey expires and in that time they can out of defense injure your snake. Feeding rats as soon as they are large enough to do so is better than feeding them a mouse of equal size as the Rat will have more available protien. Consider too dusting them with a vitamin supplement prior to feeding them to your snake. Feedings should be done every 7-10 days and the prey item should be just large enough to leave a noticable lump in the mid section...it should not look like a golf ball stuck in the middle of a 50ft garden hose! Keep in mind too that its not unusual for a snake to go off feed for a few weeks, but after about 4 weeks I would seek the advice of a vet unless your snake is a gravid female.
Hope you found some gold nuggets of information here although there is a ton more to know.