I had a few boring hours last year, and decided to play with some autocue software being manufactured by a friend, and he showed me a really cool feature that would be great for SMing.
Basically, you have a USB jog wheel, which you use to speed up or slow down the scrolling of the text on the screen. This autocue software highlighted words as it went, and by using the softkeys on the jog wheel, you could insert flags into the autocue sheet. Now if that is not nifty, I don't know what is. Imagine combining that with a tablet PC, so that you have a floorplan on the tablet, and by using the hot keys to indicate the actor, you could use the stylus to tap out locations, and viola, blocking.
Then, same deal for calling the show, jog wheel to control speed, acting as an autoprompt. Extra facilities like house lighting control could sit below the prompting window.
Line notes, as the script scrolls, push down on the jog wheel when they go off book, and release at the end of the line, auto generated line notes.
Jogwheels start from about $40
Linky, going up to the hundreds - however something middle of the road like
this would be cool... it would really be a great interface for an SM application.
I have spent a fair bit of time thinking about it, as you may guess.
As for what I would be willing to pay? It depends. A product that was an all round productivity suite, capable of multitasking etc, anywhere up to $1600. A basic database style application, $120ish.