Is there any way to reduce merchant repair costs?

Solution 1:

No, there is no method in game for reducing the cost of merchant repairs. As you said, barter does not change the price that merchants repair for. You can increase the value of repairs offered by caravan merchants by investing in them using Uncle Roe in Canterbury Commons. Each stage of investments increases the repair skill of the merchant you invested in. Costs to repair will still be exorbitant, but they'll repair to a higher quality.

As it stands, using merchants to repair your gear is almost always a worse deal than repairing the item yourself. The only exception would be using them to repair unique items that could not be maintained otherwise.