Forshaw's book is really quite scarce, I gave it a "very scarce" rating on my website. it is much harder to pin down a value of its worth. as Tarn so correctly pointed out, eBay is not a good place to determine value as there are some collectors that will pay just about anything it takes to secure an elusive volume for their collection. I have routinely seen books sell for two, three or four times above what I would consider to be a reasonable retail value.
my feel for Forshaw, is that it should be worth in the $200 to $250 range. as of this moment the bidding is already over that, running about $262 at the time I am typing this.
if you have the deep financial pockets, by all means go for it, but my feeling is that almost without exception if you are willing to wait a bit longer, a more reasonably priced copy may come along. of course, that wait might be quite a while!
hope this helps!
Michael (TheManinBlack) T