I believe "BostonRealEstate" has excellent value in most extensions and is a very pure, keyword combination ... short, clear, highly descriptive. With "Directory" added on, you begin to get into the realm of various derivatives on the pure keyword combination. It's not bad, and is still logical, intuitive. But your are then competing a little with other derivatives like BostonRealEstateGuide, BostonRealEstateAgencies, BostonRealEstateProperties, and so on.
That being said, my appraisal would be in the neighborhood of $1500. The .com is a great plus and the sheer volume of real estate pros in Boston may well lead to interest in the name. I believe the name could possibly go for a little higher. Names similar to this are listed weekly at DNJournal.com in the low $X,XXX. I think BoiseRealEstate.com went for $50,000 last year. Just imagine what BostonRealEstate.com could bring.
Developing sounds good. Give it some time. Build some traffic. Good luck.