Cheap = economy of scale.
The famous RPi SBC is a bad example to compare a headphone measuring device to. An RPi can be used for almost infinite projects, radio streamers, home automation, media servers, an actual useable PC, firewall, weather station etc, etc. What on this green earth can a headphone measuring system be used for, with exception of measuring headphones of course?
Thus it will never have a large enough community who are interested in investing in it.
That's not so say that something can't be made useful for a reasonable cost that does an adequate job, in a quick enough time frame. All terms in italics maybe wildly different depending on who is being asked. Heck even the GRAS system has tradeoffs.
If you want cheap, quick 'n' dirty, then shove a measurement mic (Behringer ECM8000) through a flat bit of foam and measure one earphone sealed. Will it be accurate? Nope, but it might point out some issues. I suppose one could then go on to take moulds of their own ears and make latex pinnae, but I don't have a clue what the interaction would be like with those kinds of mics. Would in-ear microphones be a better solution if one could make some plastic 'ears'?
Pads might not necessarily 'reduce sound quality'. They may change the tonality from stock. Whether that is to detriment or not is in the eye, or rather the ear of the beholder.