£210 to £400 is fine for a run-of-the-mill penny black cover. However, there is nothing run-of-the-mill about some of the covers on the Gibbons site or the Gross collection.
For most of the items in the Gross collection, for example, they are absolutely the best of that type of item Catalog values are irrelevant. Another way to look at it - if a "standard" cover with a "standard" penny black single has a £300 catalog value, then there is a distribution curve of values - from say £50 at one end, to £1500 at the other. It's a skewed distribution, with most of the covers between the 50 and 300 marks. The tail may be very long, but it is very thin. As a result, there is a lot of competition at that right edge among the top collectors for the things that are gems.
Look at a couple of pages of results from this auction last week. You'll see that the prices for "normal" stuff is pretty "normal," but the prices for the first two lots is a tad more (about $100,000 each, once you add the buyers premiums and convert from pounds)
http://www.davidfeldman.com/auction...all/all/all/