This assumes that the markets are perfectly efficient (something that's inconsistent with farm systems, just to name one thing).
Farm systems as well as comfort of living in a place.
For me to move to Finland to do my job would require a significantly higher pay than to do it in the US simply because I'm not moving to a country I have no connection to without a very good reason. I'd assume the same would also apply to a Finnish person looking at the US.
In regards to the farm system, it's not just current salary, but also potential future salary. It's perfectly reasonable for a player to play for $65k/year in the AHL as opposed to 250k/year in the KHL if they believe playing in the AHL gives them a higher chance of being called up to the NHL team and making 600-700k/year (whatever minimum is at now)
As far as objectively measuring league quality, it's going to be a significant challenge simply because teams don't play each other often, especially in competitive games. In a sport like soccer/football where you have robust continental competition in Europe, it's much easier to get a read on how leagues rank against each other. Sure all the teams don't play each other, but you get a decent sample to at least make a solid system.
In hockey those types of games basically never happen so you're stuck trying to extrapolate based on players who switch leagues and how they perform and there's a lot of issues with that. Does player X play better in the SEL as opposed to the AHL because the SEL is weaker, or is it because he's from Sweden and simply feels more comfortable living in his home country? If a player moves from the AHL to SEL and plays worse is it because the SEL is stronger or because he's struggling to adjust to living in a new country with people speaking a new language?