If you only plan to use the PAR meter to evaluate lights, without needing to put the sensor in the water, see the For Sale forum for a cheap one I have for sale.
I'm slowly developing a good modification to cheap Lux Meters to make them work for PAR, and under water too. I have made several, but none are as good as I want them to be. Right now I have a new prototype ready to assemble that I hope will be what I am looking for. If so, I will have 3 or 4 of them to sell later. If not, I will keep looking for a way to do this better.