thats the spirit! we really do suggest trying out the teensy or 32u4, as its smaller and cheaper and the UNO/midi thing is a real pain. for pricing, we suggest two margins to start, so lets say you include a nice laser cut grippy and a miniB USB cable (check monoprice.com for low cost cables!) - $75-$80 for a completed thing is fair given that you'll quickly be able to lower YOUR cost by buying more. REMEMBER THAT MARGIN IS CALCULATED WEIRD! 40% margin means multiplying by 1.66! so $30 * 1.66 * 1.66 = $83!
Since this is a fairly simple project, you may even start by saying something like "first 25 orders are fully assembled and tested! future versions may be kits" - this will get you quick customer feedback without the issue of kit assembly errors. what its worth, thats really impossible to know. but personally, we think that somewhere around $75-$100 is worth it for a supported, good quality, (assembled?), working airharp. the neat thing is people will be able to plug it in, and modify the MIDI messages if they want to using the IDE. thats a bonus!
and remember, if you have extra margin, and the project doesnt work out for some reason (it happens) you can always say "Clearance sale! these are now half off!" and you can recoup your initial inventory - nobody can resist a sale
for sonars, we suggest looking here for ideashttp://www.futurlec.com/Ultrasonic_Sensors.shtml
they have very slow shipping and customer service but maybe thats OK for you to start
the maxbotix doesnt use a dsp, it uses an even slower PIC from what we recall, but sonar projects are complicated. that said, if you can get it working then thats the star of the project because sonar is waaay better than IR or other light-based techinques
as you make these, you'll find ways to reduce cost but keep the quality high by adding yer ingenuity. if you open source your design, we can continue to help you find better choices for your components!