I had this discussion on HN a while ago and looked into it - the costs of transportation are high because of various logistical regulations, there is a high cost upfront cost to set up a disposal site, there is a fairly significant cost for staffing such sites, and these sites will need to be managed for millennia.

Here's a good paper on the costs https://www.mdpi.com/2076-3417/9/12/2437/pdf