MLS player earn the most as that is top tier in the U.S. and Canada. A reserve player will earn $32,500 with all other salaries going up from there.
As to USL PRO and NASL it really depends on the team. Most NASL teams will be paying more than most USL PRO teams. However, there are a number of USL PRO teams like Charleston, Rochester and Orlando who also paying decent wages competitive with most NASL teams.
Today, the Montreal Impact probably have the highest payroll and unlike MLS who control all wages from the top, the NASL and USL model is more like the rest of the world with wages controlled by the team not the league.
Players in the NASL have guaranteed wages but only get paid from the beginning of training camp until the last game of the season. In general, minor league players do not get paid very well in North America and will make anywhere from $15,000 a year up to over $100,000. But most make from $15-$30 thousand for a season. They certainly have to find other sources of income for the rest of the year like coaching.
General knowledge of lower level soccer as editor and writer of IMSoccer News. http://www.insidemnsoccer.com/