I would be careful about discussing your rate in the current climate
My companies view is contractors are paid a good rate partly due to the additional risk they are accepting by not being perm. For contract renewals or longer term renewals we often look to reduce the contractors rate.
Of course you could leave, but at my company the vast majority of time contractors don’t.
You also need to factor in what is happening to perm rates at your company. At mine people have taken a paycut this year, with another on the cards for next year.
I’d keep quiet…
EDIT – re-read your post, see you are referring to your agency and margin – I doubt it is 28% and even more doubtful they would change, but if it wont risk your contract with your current company you could always give it a go. You need to be prepared to potentially walk though.
Some agencies are very up front with their margin and make it clear to both contractor and company