Looking at buying an existing real estate brokerage. Is there a general rule of thumb in figuring how much the brokerage is worth?

To give you an oversimplified answer: it's no different than any other business. The value is, in effect, a multiple of the income you can derive from it, adjusted as necessary for its potential to increase (or drop!).
However, you are well advised to be in the business yourself first or at least to have sufficient familiarity with what it involves. In most cases, too, you have to conform with licensing requirements, which usually means a licenced manager in place.

Duncan

Unless the business keeps very good records and can SHOW how leads are generated and closed then a accurate valuation cannot be obtained.

The question becomes what are you paying for and how can you obtain a correct value?

Make sure the business or way of doing things isn't dependant on that one person.

A lot of the value of a real estate brokerage is in the equipment, good will and real property (if any). If it's a franchise, there may be some value to the use of the name.

While having a strong cadre of agents is a plus, sometimes the glue holding them to the brokerage is the owner.

There have been a couple of examples locally: In one, one of the agents purchased the brokerage, intending to keep the BIC who was employed by the former owner. The BIC left and within two weeks, so did half the agents. They had stayed because of the training and support offered by the BIC. There's another local brokerage that has an owner in her mid-70's that all of the agents love. Again, it's the training and support, because they could earn better splits elsewhere. When she retires or sells the business, there could be a wholesale exodus of productive agents.

Here's a link to a past thread where we discussed te going concern value of a Real Estate Business:
R.E. Brokerage Valuation

There is another thread regarding the value of a REO/BPO Business which I may come back and add later. Here's that one (a bit redundant):
REO/BPO Portfolio Sale