My pick for best value shotgun is the Maverick 88.
After dissasembling my uncle's 590 I can tell you that the stock, forend*, barrels, bolt, bolt carrier, elevator, and both of those little bars that lock shells in the mag tube will interchange.
The trigger group might interchange but then you would have 2 safeties and I believe that they wouldn't function together.
*the forends will interchange, but you need the COMPLETE mossberg forend to do it because it is more than one piece (websites selling forends ussually do not include the tube with the action bars, so that is why they say it wont fit the maverick).
I've put ~800 rounds through it without a single problem, which is more than skeetin 870 can say for his 20ga. (J.K. skeetin, it was those danged supersport reloads wasn't it?)
The maverick is cheap because of the cheap labor used to make it and a simplified forend. It is made in America and is a dang fine shotgun.
Happy shooting,
rugerdude