It depends on which exact version (there are several different versions of the 226). If it is in fair shape, it can be worth $300 to over $1000, depending on the model.
50-500 uSD
9mm
Best left to a SIG trained armorer.
SIG sn's are not in the public domain. You will have to call them.
12-20 rounds depending on caliber and magazine.
nothing
I cannot find any reference to marlin ever making a model 226.
No, they are two very different models in terms of magazine size.
3/4 of 226 = 169.5
226 pages worth
It depends on which model and what condition it is in. If it is a common model, 229, 226, 220, etc, and it's in very good shape, it's probably worth $500-600. If it's anything else, it depends and we need more information.
0.226 kg