A variable you have to account for is the changing SPS airdrop points changing. If you assumed all the general sale packs sold out, and no one opened the packs, it would cause around a 40% increase in the SPS airdrop points. So that alone would lower the value of the dec's returns by 40% from your current calculations.

Obviously the expected collection power from opening packs isn't as high as their airdrop value, so general sale wouldn't be a 40% reduction in dec value even if it sold out day 1 (unlikely at this point imo).

Additionally the new 2 league ranked format will double the supply of dec, which will also increase the amount of airdrop points. Lastly, Riftwatchers being released would certainly have an effect on airdrop points (not as drastic as chaos legion but even a 10% drop is a big deal for if that $100 investment is profitable or not.