Back to the original topic - The base cost thing is a balance issue but somewhat hard to quantify. The upgrade issue is a much easier problem to tackle soo...
I've been grinding on some numbers to try and suss out (or approximate) the underlying math of the cyberdeck cost/power curve. It's a nasty task with so many variables, but I'm making a little progress. The idea would be to create a formulaic cost for upgrading a cyberdecks' device rating and then further costs for adding the other features that would eventually bring the deck on par with the next tier of decks. Overall, the cost of those upgrades would be the same or a bit more than the difference in list prices between decks, but being able to upgrade in small chunks would be the real godsend to deckers, even if there was a small cost premium.
Going from a rating 2 deck to a rating 3 is about 80,000
Going from a rating 3 deck to a rating 4 is about 130,000
Going from a rating 4 deck to a rating 5 is about 195,000
Going from a rating 5 deck to a rating 6 is about 275,000
Those are steep, but definitely achievable within a career. Of course, if you made the base costs cheaper these delta would also decrease.