The PAR sensor I used in college was a couple grand. Had a big wand probably lined with a bunch of sensors.
I'm not sure how it would work with an Arduino, as you have light sensors that can measure overall lux, and then photodiodes that are sensitive to only specific wavelengths. But there aren't sensors that I know of that read all present wavelengths standardized to any amplitude you'd make sense of.
Not sure how useful you're expecting such a thing to be, you can cherry pick a few wavelengths and try to find sensors in those bands, but again without any real baseline, you'd just be able to say 'yep there is some red light in that light'.