Concerning your worries about Google Adwords:
Your argument is basically a No-True-Scotsmen fallacy. Every campaign executed poorly will burn a hole in your pocket -- almost by definition! It also applies to the other options you mentioned. However, with Google Adwords you can set a maximum amount you're willing to spend for a certain period of time. It may not convert to sales, of course. But it gives you data to think about and turn that into experience.
Concerning better ways to advertise online:
There's no silver bullet, unfortunately. The only way to find one is by collecting experiences: Spend the minimum amount of money on several different means (if possible) and measure the results. Try to figure out what works -- and if not, why it didn't. Google is quite easy to use since it allows you to restrict the amount your willing to spend. It's also quite targeted if you take the time to learn about keyword targeting.
Concerning your site:
Well, you didn't ask about it but... online advertising can fail due to a number of reasons. One potential reason may be your product page: The logo and slide show images are either poorly scaled or you didn't check if they do properly in common browsers. This makes a very cheap impression although you're trying to sell a quality product I guess. In general, the page doesn't appear to be very convincing. If you're looking for a cheap way to improve your sales, study and implement A/B testing on your homepage.
Overall, I'd say Google Adwords seems a good first option for you. You can optimize it easily since you can also use their measurement and analytic tools (check, for example, the WebSite Optimizer).
When you fixed the obvious issues, targeted banner advertising seems a better option, although you'd also need to optimize this one, too. Makeing a good banner really is an art.
Hope this helps.