It was posted on another website that Michigan Motors is selling them for $800 brand new
http://www.michiganmotorz.com/cast-iron-marine-intake-manifold-2002current-p-486.html
Differences in the marine engine output and the truck applications. Copy/pasted from GM documents
Table 2.3.2-I.
....... HP Lbs.-ft ... HP Lbs.-ft... HP Lbs.-ft
1200 54.6 238.3..... 63.7 277.8..... 75 328
1600 80.1 262.1..... 92.1 301.4..... 100 338
2000 105.5 275.0... 121.3 317.5... 125 345
2400 126.8 277.0... 147.2 321.6... 150 347
2800 152.2 285.0... 175.6 328.9... 180 350
3200 173.4 284.4... 200.2 328.2... 200 358
3600 194.4 283.6... 223.1 325.2... 230 354
4000 211.7 278.0... 241.1 316.5... 260 350
4400 227.1 270.9... 252.7 301.6... 280 340
4600 230.4 263.0... 255.0 290.8... 295 330
4800 231.0 252.7... 252.8 276.4... 300 320
5200 224.6 226.8... 242.7 244.8... 307 300
............ L30........... L31........... L31 Marine
From what I understand the marine engine is rated with open exhaust manifolds though. I don't know how much that's worth but a good exhaust/header setup should match that and probably add a little more torque?