In most cases, I am not free to send out examples of my best work as I do not have the rights to do so. Be careful to ask to see work that is similar to what you will be producing, in terms of the quality of the dancers, the venue, the lighting design and the sound situation. I, as one would tend to, try to pull out examples of my best work. Perhaps that is because I have done such a good job, but more likely because that is what their mind usually does when watching dance videos. Beyond my own camerawork, almost nothing determines the quality of the video beyond the lighting design .
Opera houses tend to come with more lighting instruments for one thing.
Don't expect the videographer alone to be responsible for the difference. If you don't know where to begin to look for a dance videographer, you can try asking some of the larger companies who they use. They consider you competition, and be unwilling to help, but most people are reasonable, and I've worked for many choreographers who were referred to me by other choreographers. Lighting designers and theater TDs also tend to know of good videographers in various budget ranges. If you city has a good dance organization, they may know of some dance videographers.