cost to install backup camera on my prius C?

Depends, if you currently have the optional Navi unit, there is a kit that multiple companies sell for between $99-$199 and you could DIY it or pay approximately 1-2hours of labor. If not, other options include upgrading to the Navi unit or doing a rear view mirror camera. Both would be substantially more expensive. I have seen a rear view mirror camera install run about $500 installed. Either way good luck with your camera. A good place to start: https://www.amazon.com/Toyota-Backup-Camera-Camry-Corolla/dp/B00IW8B29K https://priuschat.com/threads/rear-view-mirror-backup-camera.64217/

Do not do it yourself, have it done by professional

Try one of those plastic stick on wide angle viewers that you see on the back of motorhome windows. You can buy them at Camping World.. Get the smaller size, which is about $15. They work best on vertical windows , otherwise they pick up reflections. A Prius C rear window looks fairly vertical. But if reflections are a problem , hang the viewer from the top with suction cups. You'll have to poke a couple holes in the very top of the viewer for the suction cup rings. This should allow it to hang vertically and loosely so you can still open your rear hatch. I did this with my Toyota Yaris. A lot cheaper than installing a backup camera.

where do I look for an installer?

Where to get it installed in Houston?