yes it is somewhat of an upfront expense. what / how much are you wanting to grow?
to grow one or two tomato plants you'd need a bucket, an air pump, some air stones, a couple of net pots, a little media, and some nutrients. total setup might run you $50USD. Of course, that system should last you many years and you'll probably outgrow it before it stops working.
At the same time modifying your system to grow say 15 - 20 tomatoes might only jack the price another $100USD
That is all of course variable but somewhat in the range, assuming you're a decent do it yourselfer.
Testing equipment (EC, and pH) might run you another$100-$150. you can get by without it, but if you really enjoy what you're doing, you'll end up getting it.

If you're planning on growing outside, you may need to protect your plants against wind as well.
Aquarium pH kits are cheap and can be used with hydroponics (done so myself for years). Otherwise I fully support grizzman's recommendations.

As the others have said, cost depends largely on what you hope to accomplish. If you just want to grow a couple plants in hydro as a hobby it can be done rather inexpensivly.

The largest expenses for most hobby systems are the pump and the nutrients used. Outside of that, you can probably find stuff around the house or go dumpster diving for many other components you'll need.

If you'll post more about what you hope to grow, we might be able to steer you toward a certain type of system that would suit your needs and offer better figures about costs.

good luck.

http://edis.ifas.ufl.edu/hs184

Floating lettuce rafts are really inexpensive. Check out the website above, it's easy to set up and there's no need for pumps and tubing and such.

Here is a link that might be useful: floating lettuce rafts