There's an almost overwhelming amount of options today. A lot depends on what you want to do.
You probably want to hear some noise when you're playing. With electronic stuff, you'll need headphones or an amp. In addition to whole electronic kits, there are also pad sets that you can use for practice. Some have inputs for bass drum pedal triggers and hi hats.
This one is pretty inexpensive right now:
http://alesis.com/percpad
Pretty basic, but you can make some noise. They also make a SamplePad that allows you to load samples. Several other manufacturers make something similar, and some are much more advanced.
If you like using a practice pad, there are some electronic pads that have training functions - things like keeping steady beats, gradually increasing/decreasing tempo, pattern matching, and so on.
Like this:
http://www.rolandus.com/products/details/479
Other manufacturers make similar units.
If you want to practice using several pads (kit like), instead of a single pad, and don't care about hearing electronic sounds, you can use something like this:
http://www.dwdrums.com/smartpractice/specialty.asp
If you ever plan on using an acoustic kit, that's the way I'd go. You can use mesh heads for quiet practice. You can install triggers in them with mesh heads, or on them, with acoustic heads, and some manufacturers make units that can sit right on top of the heads. Like this:
http://pearldrum.com/products/kits/electronic-drumsets/tru-trac-drumheads/
With most of these options, to hear something, you'll need a module, and an amp or headphones.
Of course, there's always whole electronic kits. They run from very cheap to very expensive. A lot of people really like them, but they never interested me much.
There's lots of options, depending on what and how you want to practice. Any of them can be bought new. Buying used will save you some money, but there's different things to look for with each.