Also, most public defenders, prosecutors, and court functionaries are not "spending someone else's money." Rather, at least in many metropolitan areas, they are salaried government or non-profit employees who are just trying to get the justice system to work.
On public defenders in particular (our first target market) I'd recommend reading a few of the NYT's op-ed series from this year which have good insight on how starved for resources and time much of our potential client base is: http://www.nytimes.com/2016/02/19/opinion/when-the-public-de... (New Orleans) http://www.nytimes.com/2014/02/19/us/public-defenders-turn-t... (Missouri) http://www.nytimes.com/2016/04/30/opinion/a-mockery-of-justi... (Fresno CA, South Dakota, Minnesota)
The folks we interviewed have said if you can save us x amount of time we would be thrilled to pay you $y per case because it will be more than worth it. We will only be able to see how much the market will bear once we're done building our product but, so far, the fact that we have been able to get a public defense firm to sponsor some of our development costs bodes well for the idea that lawyers are interested in spending on tech that saves them precious, precious time.
EDIT: I'd encourage everyone to read through the application posts. I think the Q&A in our thread fleshed out quite a few of the concerns about starting a business for this particular subset of the legal profession.