Boring question #29: How accurate are translation software and programs?

Every so often I need to use an online translator or some sort of translation program, to translate something on a Web page to English or to translate something to someone I am corresponding or chatting with over an IM. How do I know if the translator is really working? If I am speaking to a person via the IM and they know enough English to tell me I didn’t say what I thought I said, then I know the translator is slightly off. If it is a Web page or email there is no one there to give instant feedback so how would I know if it is doing the job? I have thought I could test it by having it translate something then translate it back to the native language but wouldn’t that be a poor test? There are certain words and phrases that don’t carry over from English into a lot of languages but I am sure the translator does its best to fill in the gaps. If I were to translate the translation back into English, I would just be translating the version that the translator did with the assumed or filled in words and phrases and thus I would have no ideal if it was working? Does accuracy come with spending more money? If I got a $45 translator it would work better than a #20 translator but not as well as one that coast $89?