Providing agencies with white label work

Are these local web design agencies? Why don't you open your own web development business? You have a portfolio, you should have testimonials from businesses on the sites you built (since you worked with them directly) and could charge your own rates. As a subcontractor for "web design agencies", you are delivering a core service/product these "agencies" are unable to deliver using their own skills and talents. They are not directing the scope of work, site planning, creative, client communications or anything else in the web development process?
As long as you are doing the work and they are re-selling it, you are on the short end of the money, have created a J.O.B. for yourself and are at the least desirable position because you constantly have to compete on price against all kinds of low priced providers.
PLUS you have to haggle with the "web design agency" for a fair share.
Here's a thought ... Flip the tables and generate your own leads and then you can make/keep the client payments as your own. (Unless of course you are very comfortable and satisfied with the role you are in now.)
Want to stay as a contractor? Set a firm percentage you want to provide the product they are selling, show the unique benefits you bring to them (unquestionable ability to satisfy clients and manage the entire process so it costs them 0 to deliver the product) and stick to your percentage. You'll want them to verify the website price charged to the client. You bring a whole lot more to the plate than someone who implements instructions and is not involved in the time-consuming, talent-driven portions of a web design project. They'll have a nice ROI.
BTW we have a marketing and business development firm. Our virtual teammates follow our instructions on customizations, css edits, install content and images we create, on the theme we provide, using the plan and site map we develop, and we carry all the back and forth with the clients. We pay between $300 and $400 per site, depending on how much work is needed on their part to complete the site as we plan it. With us, websites are almost always part of a total "marketing makeover & profit focus agreement" that's within a 6 to 12 month contract so we are not a "web design" agency like those you deal with. Cheers to your success!

Sounds good Please PM me about your services I have a quite a few of people interested in this type of srvices.

In the other hand, 40% in my opinion appears to be a decent percentage in my opinion as long as you white label your services.

regards