I'll go out on a limb here and say the putter is the most important club in the bag, regardless of cost. As long as you have something that fits and works for you, then it doesn't matter what it costs. That said, because you use it on EVERY* hole, it seems to me it makes sense for it to be the most expensive club in the bag,
There are guys out there who will buy a $500 driver and upgrade with a $500 shaft just so they can add 3 yards to their 213 yard drives but won't spend the money to get fit for a new putter instead of the beat to hell, old fire poker they currently use. Meanwhile, they 3 and 4 putt 13 greens and take 7' gimmies.