[Radim]

1) Are you doing something innovative? By definition there's no clear value to baseline against. Clients don't know what's even possible, much less how much $$$ it might bring them. So the value in "value-based" is correlated with some internal strategy / personal / brand politics. Good luck quantifying that as an external SW dev firm.

2) Doing something run-of-the-mill? Improving a process that's already in place? The client may have an idea about its ultimate specs & ROI. Even if they're able to articulate those to you – why would they?

In practice, value-based pricing comes down to "charge as much as they're willing to pay". Which is fine, that's the essence of sales. But more to do with social skills & play-it-by-ear, rather than some objective numbers you might hope to extract from a client.