Let me start out by saying I have no yellowbelly complex in my collection so I'm by no means an expert. I posed this same question to Sean @ Exotics by Nature. This is his answer copied from his facebook page.
" Clay it all comes down to what you produce. With a highway you will produce both YBs an Gravels BUT sometimes these can be hard to spot so you are paying more money to not have the YB involved. We didn't set this price structure but we believe in it."
According to Sean prices are as followers
highway 7500
gravel 10,000
super gravel 20,000