That's right, but the payment rates depend on average on the market. When is was SEO motivated people could get pass-through links on other sites for several bucks, so the most SEO attractive forums could bring you the same amount maximum. Of course, like LoyceV said, it was not on every forum, many added rel=nofollow tags on signatures, so you could earn on your signature mainly on the forums which wanted more content which would be paid this way. All those forums are dead already because that era of mega popular SEO links is over more than a decade.