At RockingBookCovers.com the prices are from $50 to $150 per cover. They vary depending on complexity of cover design.
In general industry overview, prices vary quite a lot. The quality ones start at around $30 and go up to $400. Lower price than $30 is crazy and wouldn’t be worth designers’ time to create.
More expensive premades are those that require digital painting skills and/or 3D work and they easily are worth $150-300+ per cover. Most common in Fantasy, Young Adult, Paranormal covers. Prices are higher but covers often look superb!
Lower priced covers $30-150 are more common in genres where covers don’t need to be super elaborate. Genres like Romance, Thrillers are perfect examples of mostly photomanipulation-only covers (cutting and putting together image elements to create the cover). These usually don’t involve a lot of digital painting and take less time to make. Thus, lower price range.
In general, Premades are an awesome option for indie authors/small publishers to use because you can find premium covers for non-premium custom cover prices. More and more designers are doing them and creating more and more supply so you are quite likely to find something good.