The easiest way to tell the exact age of Remington guns is with the 2 letter barrel code typically found on the left side of the barrel near the chamber.
http://www.remingtonsociety.com/rsa/questions/barrelcodes
On shotguns it can be a little tricky as the barrels are easily swaped from one gun to the next but the barrel date can be cross referenced with the following:
Model 1100 LETTER PREFIX
1963 TO APPROX. 1968: NO SERIAL NUMBER PREFIX
1968 TO PRESENT: LETTERS USED (IN SEQUENCE)
L-68, M-74, N-78, P-85, R-90
Model 1100 LETTER SUFFIX (DESIGNATES GAUGE)
V 12 GA. (2 3/4”)
M 12 GA. MAGNUM (3”)
W 16 GA. ( 2 ¾” )
X 20 GA. “HEAVY FRAME” (DISCONTINUED)
N 20 GA. “HEAVY FRAME MAGNUM” (DISCONTINUED)
K 20 GA. LT-20
U 20 GA. LT-20 MAGNUM
J 28 GA.
H .410 BORE (2 ½” OR 3”)
I would have no issues paying up to $550 for one that is in excellent condition...some will say that is too high but I'm biased towards older guns that have a well proven track record and were built in a time where quality of materials, workmanship and pride was found other than in a dictionary!