It is still a collectible book, but not like the first Black Sparrow editions. You'll see UK firsts of Post Office (London Magazine Editions), Life and Death in the Charity Ward (London Magazine Editions), Factotum (W.H. Allen), and Women (W.H. Allen) on ebay fairly regularly for stupid asking prices. I got the first three of these about two years ago for just over $200, which is what some folks are asking for Post Office alone. I feel that I paid a bit under reasonable market value.
If yours is as clean as you claim but has a price-clipped jacket, you're still looking at a book that's worth in the $40-$60 (US) range, in my opinion. There's a copy on abebooks right now in the UK that is very good (DJ worn, book read) for $55.68. Yours sounds nicer but is price-clipped, so it's somewhere in there, although abe prices are about 20% above market value if you ask folks 'round these parts.
If you're in the U.S., there's a very limited market for the UK firsts (I happen to like them well enough (particularly Life and Death...), but not overly).