The recent D&D Heritage Auction that just ended Jan 16th, paints a healthy market for graded sales. I took the liberty of posting the top 18 graded module sales. You'll notice that the 15 of the top 18 were all in shrink. From a purely investment standpoint, paying $20 to get a module graded really has the potential for a huge return on that investment.
R1 To the Aid of Falx CGC 8.5 A Seal Grade $3,360
D1-2 Descent Into the Depths of the Earth CGC 9.6 A Seal Grade $1,680
UK3 The Gauntlet CGC 9.8 A+ Seal Grade $1,320
R4 Doc's Island CGC 7.5 White pages $1,200
B7 Rahasia CGC 9.0 B Seal Grade $900
G1 Steading of the Hill Giant Chief Fourth Printing CGC 7.5 A Seal Grade $840
R3 The Egg of the Phoenix CGC 7.0 White pages $780
CM8 The Endless Stair CGC 9.4 A+ Seal Grade $660
CM9 Legacy of Blood CBCS 9.4 White pages $600
UK4 When a Star Falls CGC 9.8 A++ Seal Grade $600
WG5 Mordenkainen's Fantastic Adventure CGC 9.0 A+ Seal Grade $576
M4 Five Coins for a Kingdom CGC 9.6 A+ Seal Grade $576
B4 The Lost City CGC NM+ 9.6 A Seal Grade $552
I1 The Secret of Bone Hill CGC NM 9.4 A+ Seal Grade $552
Q1 Queen of the Demonweb Pits CGC 8.5 A+ Seal Grade $552
X1 The Isle of Dread Second Printing CGC 9.6 A+ Seal Grade $528
M3 Twilight Calling CGC 9.8 A Seal Grade $504
C2 The Ghost Tower of Inverness Second Printing CGC 9.2 A+ Seal Grade $480
- Richard