The power rests in the bodybuilders hands, if they want to get paid more they will have to be willing to delay immediate rewards for bigger later rewards. In other words willing to, in unison step back from competing or repping companies until they get more money.
Of course this might back fire as supplements companies can use other types of athletes to promote their product, with the high likely-hood of greater sales success. Of course there is a direct market for guys that want to get huge.This might also back fire because their will likely be other up and coming bodybuilders ready to compete or rep the supplement companies for the same or less cash.
Many bodybuilders do not compete for money, at least initially, I would say the main reasons are for fame, glory, trophies, ego, etc.
Then there are bodybuilders that compete to overcome some self-esteem issue in their past. For these bodybuilders they don't compete for money but for acceptance. This type is probably more likely to be manipulated in the game [gfp, excessive anabolic use, etc..]
the business smart bodybuilders, the ones with the higher self esteem tend to make better business decisions, and not just go for getting on a magazine cover as sufficient payment. Magazine coverage is fine initially but after a while, most would like to get paid for their efforts. It really comes down to self esteem and what the bodybuilder will except and what he/she things they are worth.
Bodybuilders with higher self esteem and more business savy like Yates, Cutler, Coleman, Gaspari, Labrada, Murray, etc. seem to make better decisions. If more bodybuilders studied business and stuck to their guns then the supp companies would be more likely to pay higher. The supp companies probably don't respect most bodybuilders.
If bodybuilders want more respect they will have to demand it and if it's not gotten then be willing to step away.
Another important point is that other pro athletes [yeah you don't think that bodybuilders are athletes]
have agents that help them negotiate their contracts. Maybe that is what bodybuilders need but it unlikely as the money in the sport is lon the low side right now.