Any of the lifetime signed and numbered serigraphs in excellent /very good condition seem to be getting anywhere from $400-$1000. There are exceptions to every rule however and some images are simply more popular than others. For example two signed Casa Lupita recently sold on Ebay in presumably excellent condition for about $500. Another for $400 for with some matt burn. This is a loved piece and typically harder to come by but the market showed us the correct price. (I'm sure Rob will scream here because he loves it!) There is a Casa Lupita on Ebay over 1k that has been sitting.
An excellent "Mirage" State Edition also sold in the $500 price range, usually getting more but I think there were just too manysale at the time. That is a premium piece and can get more.
I personally have sold two #1s that were mint for $1200 but that is because the buyer wanted the #1 and the two I sold were premium images. "Mirage" and "Mother Earth's Paris," as pink as can be.
A "Park South" might get a little more due to popularity ($700-800). A "Texas" (probably the most popular due to the Duran Duran album culture) I have seen get over 1k but that is not typical in my opinion.
It really depends on how many are available at any given time and definitely its condition is most important. (Was it Previously framed, are their scuffs in the black hair? Is there any fading?) That piece is hard to find without scuffs in the black. Also know that the "Playboy 13th Anniversary" piece also had a very large signed in screen edition (more than double the average 1200 sis releases). Playboy had 2500 unsigned so it is an easier piece to find if someone just wants the image and does not care about a signature.
I know this is not an exact number but if you are considering selling throw it out to our group and see what comes in or put it up on Ebay say at minimum reserve $400 (I wouldn't sell it for less if Mint) or so with an auction and you will get a real buying price. My advice is ....Just do it when there are not multiple listings of the same piece, under 1k.