Hello John,
Just wanted to mention that I have done what you are after. I too use Arduino (Pro Minis) as Accy Decoders, the Geoff Bunza implementation from MRH (). I do not use Goeff's boards, although I do have some. I created my our that do not have the two motor outs and support easy connection of multiple servo outputs.
I use these to control SG90 servos for each turnout and the panel indication. In JMRI each item is created as a Turnout on the Turnout table. This gives you control from JMRI. I them created layout panels in JMRI and physical panels on the layout. For inputs (pushbuttons) I used the inputs from my DCC++ system (the spare ones from my Mega). Now changes on the physical panel are reflected in the JMRI panels and vice verser. I built a technology demonstrator for the guys at my club that has all these elements on it. Here is a photo:??
Using these you should be able to stay well under the $5 budget
Dave,
Brisbane, Australia
Dave Lowe