I will go down to a few hundred exacts for core B2B terms. The B2B market just doesn't get the volume of searches that the B2C market does. For example, a piece of industrial or laboratory equipment might only get 250 searches a month. But when that piece of equipment A) has no obvious substitute (it is must-buy equipment for certain types of business) and B) is a high-ticket item, then I imagine that owning an exact match domain even with such few searches will have benefits to the companies that manufacture/sell/lease such equipment. They may only need a couple of hundred visitors a month to generate several valuable leads, after all...
Of course, I also check to see that it's not a single-source market i.e. that there are several competing firms making the same product (or offering the same service - this applies also to specific services).
In fact - and while every domain has to be judged on the specific merits of the niche it relates to - I believe there can be a lot more value to a B2B domain with 500 exacts than a B2C domains with e.g. 2,000 exacts. The latter is a relatively unpopular product given that it's selling to "the masses".
