At the moment the only way to retrieve payload from the ISS is via the SpaceX Dragon capsule.
Previously the Space Shuttle admirably served this role, with the ability to take large and heavy payloads back, up to 32,000lbs (center of gravity/balance limits greatly limit this in practice).
Dragon can return 5,000 lbs of payload, so significantly less. (Beyond LDEF, STS-51A two satellites, SpaceLab, MPLMs, not that much big was ever returned).
Soyuz can return on the order of 100 kilos max, and depends on the mass of the astronauts returning, so functionally almost nothing.
Space Shuttle costs are next to impossible to reasonably calculate. Consider the range of costs assigned to flights that can range from \$300 million to \$2 billion. How you would break that down for return payload? Likely not possible.
Dragon's contract is only for upmass, per se. The downmass is icing on the cake.
Now if CASIS is providing your transit to the station, the question is, would they offer you a return cost?
The RFI for the second Commercial Resupply Services (CRS-2, not to be confused with the CRS-2 flight by SpaceX, nor the CRS-2 mission flown by Orbital).
What is interesting is that CRS-2 expects vendors to provide almost equal return or disposal services as to launch services. Thus for CRS-2, the commercial providers are bidding craft that can launch 14,000K a year in 4-5 launches, but also dispose of or return 14,000K of cargo in return.
More interesting is that it acknowledges that either the CBM or PMA may be used for berthing or docking, and that plans need to explain how to handle large cargo.