It depends on the sponsor and the athlete level. For example, a ski sponsor might just give you free skis, they might pay you - it depends, but a different sponsor (like target, red bull, bigger companies) will just give you cash and some swag (like free red bull). It's all based on marketability, how much money can you make for the company, if you are in contests, winning a lot, have big parts in videos, and are able to put in ads, you'll make more. It's like any other action sport, if you're a big name (Tanner Hall, Simon Dumont, Ryan Sheckler, P-Rod, Dave Mirra, Travis Pastrana - these are all names know by people in and OUT of the sport) you'll make more money. So the key is to put yourself out there and make people notice you - like T-Wall is currently doing - he's one of the biggest names in the ski industry right now, but outside of the winter sports community (cause snowboarders know him too) people are like Tom who?, but he's becoming a bigger name outside the industry by competing, making good video parts, and he is marketable, people will by stuff if he endorses it (how many people in gear talk ask what outerwear, or skis, or goggles, or helmet, or whatever T-Wall has).
No, I do not know this from personal experience as I am not sponsored, but that's basically how all endorsements work cause I learned about them in marketing class.