King Reza: Inexpensive means "not expensive". Cheap also means "not expensive", but can also mean "of poor quality". When we describe a person as cheap, we mean "stingy, reluctant to spend money".

The prefix in- means "not" in words like inexpensive, inappropriate, inconceivable, etc. Sometimes this prefix undergoes assimilation, so we get words like immaterial, impossible, illogical, irresponsible.

Valuable and invaluable have similar meanings, not because the prefix in- somehow intensifies the word, but because they are based on two different uses of the verb to value. To value can mean "to regard as having value", as in a sentence like "I value this opportunity." If we say something is valuable, we mean that it is worth a lot. But to value can also mean "to assign a value to", as in a sentence like "The stolen merchandise was valued at $3000." If we say something is invaluable, we mean that it is so precious that it is impossible to assign a value to it.

There is also another prefix in-, which comes from the Latin preposition in, meaning "in" or "into". This occurs in words like inhale, inject, intense, implicit, and inflammable. Inflammable means "capable of being inflamed".

The Oxford English Dictionary has an entry for untorn. It's not a word you're likely to use every day, but don't let that prejudice you against it.