Brand new OEM units are about $600 each which is a great deal if you consider that they are brand new and come with brand new Internal wastegates and wastegate actuators. You can rebuild your CHRA's as well, for about $250 or less, but are still left with a rebuilt unit with potentially very worn wastegates. All in all, if looking for on the cheap, I'd just bite the bullet and get brand new OEM units from Tischer or the like. Some would even be interested in your cores for maybe a couple hundred and the condition in that situation does not matter.
On another note, there are other things that can contribute to smoking out the exhaust, but turbos is definitely a possibility as well.