As has already been said depends what you are looking to do & what sort of machine you have, if you are buying a 4 axis machine they are generally commercial packages & come with the required software, if it's a 3 axis machine then there are a lot of people selling cheap machines that come with nothing. Just remember you get what you pay for generally speaking so if your buying a cheap machine (in relative terms) it will be far more limited in the type of work it can do & the materials it will be able to machine.
If you don't already have any software you can use then a package like vcarve pro that Rick suggested is a good option, yes you can get cheaper but Vetric seem to have a very good support system in place, the tutorials are very good & they also have their own forum which is suppose to be very good, it's actually very reasonably priced for what it is & what it does.
What your planning to do with the machine will also be a factor, if your just looking for something for your own use & have loads of free time available then a different option might suit you better.