The majority of headhunters are compensated on commission, taking a cut of your first year's pay when you land a job. Thus, their rate will also increase as your income does.
Contrarily, recruiters employed by recruiting agencies are compensated with a regular salary and occasionally a bonus (don't worry; in both circumstances, their pay is covered by the firm hiring them).
However, this frequently works in your favour.
Before it doesn't. When they are unable to discover you any other viable prospects, they may occasionally push you to accept a poor offer.
For this reason, you must inform them in advance of the wage range and threshold you are seeking, emphasising your non-negotiable demands. Never succumb to pressure from anyone, not even hiring managers, to accept a job you're not happy with.
Pro Tip: Honesty and professionalism are always valued by employers of headhunters. Your interactions with job agencies will directly reflect how you interact with everyone else. If you create a good impression on them, they'll be sure to spread the word to the hiring business.