ynneb - Benny, I vote for the less than 70 bucks - seems thats where I'll be when I finish mine up. The IPAC I bought was like 35, add a box and I cobbled a set of switches using old computer keyboards - (I was gonna hack an approach - but went IPAC cause of ease of pNp. Like the membrane idea. Radio Shack (in US - maybe Dick Smith downunder too) has a box and multi-button membrane keypad bundeled together and pretty inexpensive - maybe less than 15 bucks - so you could get there fairly inexpensively.
I like the approach (image) on the "Tren-Tec" site that advertizes on the 'zone. The sorta circular layout (on their software) helps show the z axis as I recall. That seems to be the biggest hurdle - how to 2D display the 3D layout - without getting too confusing. Best of Luck Jim