... but then I went 1-2 in the next draft and found I didn't get any gem rewards at all. That surprised me. Still, I did get five packs worth of cards for 1500 gems, which isn't terrible.
That is exactly the reason why competitive draft doesn't pay off for the average player. For 1 win you get 5 Packs and 1/3 of a wildcard for the pack opening. On the other hand, two quick drafts give you at least 8 packs, 1/3 of a wildcard and 100 gems – guaranteed for 0 wins and it gets better from there on. So unless your winrate is extremely high, quick drafts always offer better rewards. Even an average of 2 wins in competitive drafts requires a winrate over 66 % and the rewards for that are not even better than the rewards for the same win percentage in quick drafts.
Yeah I thought it was 500 gems for 1 win, 1000 gems for 2, 1500 gems for 3, which would've smoothed things out. Still, in the 1-2 run I did get 5 packs worth of cards for 1500 gems invested, which is well above the rate when buying from the store, so I'm not complaining. My experience with quick draft is that I average about 4-5 wins, which isn't sustainable in the long run either.