A couple of people have asked me what this would cost to build.

The answer is if course the usual, "it depends".

For a rough idea I just added up the bill of materials listed here for the exact prototype I built.

When all parts, plus a box are ordered off Aliexpress I come to a total of 56 Euros. If you already have stuff lying around it will be less. I know I spent a good deal less for the prototype because I already had switches, prototype PCB, resistors and capacitors etc.

 The most expensive items are the MAP sensors amounting to 50% of the total cost. The second most expensive item are, hold on to your hats; the four push buttons! I calculated the price using decent push buttons you would mount in a case, not the fidgety PCB mounted ones I have on the prototype. The Arduino modules are cheaper than the Mega328p ICs cost separately. They are in fact dirt cheap, as are the displays.

Now I didn't factor in the soldering tin and odd bits of wire you need. I assume you have a decently stocked electronics setup. If not, you may need to add the costs of a soldering iron and a pair of side cutters etc. In that case it will be as expensive as you want to make it. OTOH, this may be just what you need to reinvigorate that old electronics hobby!