How much do online marketing agencies charge large companies?

This is a question that can't be answered as given. It could be a few thousand dollars a month or several million dollars per year. It depends on the services offered, how the company bills the client, and how much the company is willing to pay. Also, you have to realize there aren't any set definitions for things like "established agency" or "large company." These mean different things to different people. Is a "large company" Coca Cola or In-N-Out Burger... obviously the two aren't anywhere near close in size. Make sense? Rephrase the question and you might get more definitive answers.

It really depends on the client's needs. There's really no single ballpark figure we can give you to cover all those cases.

The answer below is complete BS, coming from the agency world and seeing first hand. Quality and "size of the agency" or even how big their clients are has almost an inverse correlation to effectiveness. The smaller agencies often have the more skilled, exprienced and responsive staff. While large agencies get overloaded and end out outsourcing to dispensable contractors, the larger the agency often the more disorganized and more disconnect between the technical ones actually doing the work for the client and the salespeople actually doing the communication. oh and for a large site expect 50,000-1,000,000,000 USD.

One thing that will change as the client size grows is man hours; the bigger the client project, the more time is allocated. Typically the bigger clients will have more content to create, optimize, etc. their mailing lists will be bigger etc. (thus incurring more costs), if you are going to offer paid search or display advertising as part of your service then budgets for these can grow into the millions (of which the agency typically takes a cut for management). The other aspect of the costing process is the value of your staff; if your staff are highly experienced, skilled and trusted, their time is likely worth more than another agency dealing with smaller brands. So after you've worked out how much time a client account needs (based on their activity) then multiplied that by the value of the time (some well known agencies can be $3000+ per working day) and added in the spend (ppc, adwords, retargeting, email marketing, user testing, etc.) the total bill is pretty large. This should help clear up a little, but as the others mentioned, it's dependant on many factors both agency and client side. I'd imagine an easier way to find out specifics would be to look at publicly traded companies' accounts?