My recruiter was quite open with me.
The company he worked for charged 15% of my salary. He himself took a percentage of that which increased at stepped intervals when hitting monthly targets reaching almost a third of the fee.
Surprisingly advertising jobs seemed to be one of the biggest costs accounting for about £1-2k per placement. Office, transport and other costs obviously had to come out of the fee too.
Overall he had about a £20K base rate and was aiming for £20-40k in commission each year.