First, I must say EXCELLENT! I haven't had the opportunity to use your interfaces yet, but you may consider adding LED support if it isn't already included in your current interfaces. I'm guessing that you already have this, but it was missing from your list.
As for the pricing.... I'd say 90% of the mame cab'ers fall into 2 categories:
1.) Basic - 2 joysticks, 6 action buttons or less each, and the regular P1/P2/Coin buttons.
2.) Extreme - Everything they can fit onto one (or multiple) control panel(s).
There's the occasional single stick project, cocktail, convert a jamma & add a trackball/spinner project, etc. but those are definitely the minority.
Obviously, your new interface will appeal more to the #2 type builder. A combination of comparitively priced interface products that give the same/similar device controls puts you in the neighborhood of $180 or so. I'm sure you've done your homework and are aware of what is already out there and what they sell for.
It's not likely that you can get your price down to where the #1 builders would be tempted to go with Ultimate Interface rather than a MK40 "just in case" they want to add more devices in the future (I'm sure there are those that would, but I believe them to be a great minority). So, if you undercut this magic $180 number too much, you are just cutting your own throat, so to speak, and losing potential profit from the #2 builders.
On the flipside, if it is too much more than this $180 number, then the #2 builders will most likely go with separate interface components. However, you are offering something nobody can compare to: a single USB interface. Now that is something that people may be willing to shell out some extra $ for.
I'm curious how a PC treats multiple devices using a single USB interface. I mean, if two trackballs are connected/detected as a single USB device, can MAME distinguish between the two? Would one be automatically assigned to P1 and the other to P2? Would a special mame derivative/port be required, or would your interface take care of everything? Ahhh, so many questions you have sparked in my mind!
Great work, Ron! I'm sure this will turn into nearly the longest thread on the message board so far.