With the experience of transcribing for FreeBMD to back me up, I can vouch for their having double, if not triple, quality control.
And yet there are still errors....
FreeBMD aim to transcribe every entry twice, and if you've ever wondered why some entries are in bold, that's because they've been transcribed twice with exactly the same result. FamilySearch operated a similar system in the past, but went one step further - if the two transcriptions differed then a third person would mediate (they may well still use the same system).*
Both of those are volunteer projects - the cost is not monetary - instead the cost is a delay in publishing the records. FreeBMD transcribers have transcribed 322 million entries, but 70 million of those are duplications - and every duplicated transcription could be said to represent an entry that hasn't been transcribed at all. As the FreeBMD project has been going about 20 years, it could be argued that the duplication has delayed the project by about 4 or 5 years.
To summarise, there is a substantial cost involved when records are transcribed twice: at best it will cost twice as much (commercial project) or take twice as long (volunteer project) - and still some errors will slip through. When images of the original documents are available online the transcription is merely a finding guide - so long as researchers can find the records that they're looking in a reasonable time for it doesn't matter how many errors there are in the transcription, because most people are going to give it only a brief glance before transferring their attention to the image.
* When FamilySearch were double-transcribing the IGI they didn't have
any online images, so the accuracy of the transcription was crucial. Similarly, at the start of the FreeBMD project they didn't have online images of the GRO index pages.