"Just think about ["about" is necessary here] this: if you buy an old rope, will it be expensive?"
THINK with no adverb (about)=imagine. THINK with adverb (about)=ponder, consider, evaluate.
Implied moral: A new rope may be expensive, however the safety it imparts will be more than worth the extra expense. An old rope may be cheap to buy, but cause expensive damage when it breaks.