How can we determine the value of a "technical story"?
As discussed below in the case for relative value: The value of a story derives from making the system more attractive for end customers. And that could either be developing a new feature or making the system scale better.
For technical stories that fall under non functional requirements (requirements being the key word here), it should be possible to establish their value based on the above definition. An extreme position would be to de-prioritize any NFR / story that cannot establish how it would make the system more attractive for end customers. There is a possibility that inadequate articulation of the value may lower the perceived value of a tech story. But hey, I would ask the tech guys to articulate the value in a different and better way rather than let it get in the back door.
Note: Value need not necessarily be limited to returns in the short term. Long term benefits should also be considered to be of value, in some cases, more valuable than short term gains.