In terms of sheer numbers, collectible softcovers are vastly outnumbered by collectible hardcovers. However, many paperbacks - books with soft, not rigid, paper-based covers - sell for high prices. The reasons vary - authors self-publish, publishers lack the necessary budget or the desire to invest in a particular author (think of poets particularly) or simply softcover is the format of choice for the genre (think Pulp and the vintage Penguins that we see in used bookstores).
One of the challenges of collecting important softcovers is scarcity - they simply do not last as long as hardcovers and rarely remain in good condition. But that simply increases the thrill of the chase.
Penguin will always be associated with paperbacks as they popularized the format via clever branding and affordable pricing that appealed to a mass audience. Their first 10 paperback reprints have gone down in publishing history but they don't attract high prices. Ace is another example of a publisher that truly mastered the paperback format. Many influential photography books are also softcovers.