Combining would be good but - numbers can be tricky here as in your formula.
You would need 10 billion lvl 1 decoders to form a lvl 10.
Despite from this unpractical number - either lvl 1 decoders would becoe even worth less then nothing or higher lvl decoders would increase in value very much and allow market speculation and create pseudo NIC.
Optimum would be to choose a system that would keep the current decoder prices at the current prices, but still required some work when combining decoders for an upgrade.
This here - is a more realistic approach and would keep similar current prices without market inflation:
x = (1*(x-1)) + (2*(x-2))+1 + (3*(x-3))+2
where x is the decoder lvl you aim for.
This would mean - to make a lvl 10 decoder you'd need:
1 lvl10 = 1 lvl9 + 3 lvl8 + 5 lvl7
the others would be made of:
1 lvl9 = 1 lvl8 + 3 lvl7 + 5 lvl6
1 lvl8 = 1 lvl7 + 3 lvl6 + 5 lvl5
1 lvl7 = 1 lvl6 + 3 lvl5 + 5 lvl4
1 lvl6 = 1 lvl5 + 3 lvl4 + 5 lvl3
1 lvl5 = 1 lvl4 + 3 lvl3 + 5 lvl2
1 lvl4 = 1 lvl3 + 3 lvl2 + 5 lvl1
now to start from scratch and only start with decoders lvl1-3 you would need:
1 lvl5 = 4*lvl3 + 7*lvl2 + 5 lvl1
1 lvl6 = 12*lvl3 + 16*lvl2 + 20*lvl1
1 lvl7 = 29*lvl3 + 55*lvl2 + 60*lvl1
1 lvl8 = 85*lvl3 + 133*lvl2 + 145*lvl1
1 lvl9 = 232*lvl3 + 378*lvl2 + 425*lvl1
1 lvl10 = 632*lvl3 + 1052*lvl2 + 1160*lvl1
i think this is probably a superincreasing knapsack, correct me if im wrong.