Turbos range from $300 for a Chinese clone to $3000 from a Cummins dealer (and you need to prove to me that they are not identical!). I have had great difficulty matching the numbers on my 6CTA version and as above, huge variations in price. Definitely avoid a "marine" store if you can and get one from a farm specialist or a truck shop. If it just needs bearings and seals, a turbo shop can do it, if it needs wheels and shafts, you need a cartridge which is harder to find.
Why do you think your turbo is shot? Smoke? Poor response? Sometimes a good cleaning of the intakes etc. is all they need. Clean air filter? No restrictions in the CC breather?
Don't believe everything that you think.