Cheapest accurate PAR meter can be build from a wired Apogee SQ-120 sensor(only sensor 153$) and a cheap multimeter to read out the sensor. The sensor works with millivolts so you need to set the multimeter to 200mV. The readings must be multiplied by 5 and for LED's you need to apply also a correction factor of 1,08.
So if you see 50mv on the meter it means 250μMol/s/m² and when using LED it would be 270μMol/s/m²{250 x 1,08=270).
Compared to the new SQ-500 the measuring are within 2-3%. Costs ~165$!
Its actually pretty easy to built cuz you need only to connect the black and red sensor wires to the according inputs on your multimeter.(the white sensor wire should be insulated because the multimeter is not able th read the additional signals of this wire. But it works without.)
There are a few videos on youtube and it seems to work very well. But I will stay with my good old lux meter because it does the same and with the correct conversion factor I can get pretty accurate numbers.
Cheapest PAR meters use the same type of lux sensor and calculate PAR from lux so whats good enough for them is just good enough for me.
I'm waiting for a cheap spectrometer handheld for around 200 bucks.
It can also be used to calculate PAR but it can show you so much more. Or something like this but this little thingy needs a smartphone with a single optic cam and the corresponding app to work. No fancy dual or quad sensor cam...
https://www.findlight.net/light-detection-and-measurement/spectrometers-spectrographs/spectrographs-and-monochromators/gospectro-alphanov-smartphone-based-light-spectrometer