People don't value free products.

It is true that free ministry resources are at times not valued as highly as paid ones, and assumed to be of low quality. However, that is only because so few people give freely that such a culture has been established.

People also associated free software as being low quality and yet now some of the most popular and respected apps are free! Google especially changed the culture so that now their brand is associated with high quality even though many of their products are free. Google of course monetizes their products via other means, but the point remains that they were able to change the culture and perception of free software.

Likewise, the more people start to give ministry resources for free, the more they will be associated with quality rather than just being the leftover resources organizations failed to sell.

But to really blow this objection out of the water. Jesus' ministry was free, Paul's was free, and the gospel is free. Should we charge for the gospel to make it more appealing to people? Obviously not.