Taking a simpler mathematical approach, we can just determine the expected value of a new quest and decide based on that value.
For this calculation, it's important to take into account the quests' cooldown of 4 days. That is, once a quest has been completed, it can only reappear 4 days later at the earliest. Quests completed in the last 3 days as well as quests currently in the quest log are thus unavailable as the new quest (this includes the one we're rerolling).
As far as I know, all quests appear with the same probability, which is 1 divided by the number of quests available if the quest is neither currently in the quest log nor on cooldown, 0 otherwise. The expected value of the new quest is thus the sum of all the rewards of all available quests divided by their number. There is little research on this (most of which has statistically insignificant sample sizes), so this assumption might be wrong.
This is a list of all currently available daily quests:
- 40 gold: 9 win 2 with [class] or [class]; 9 play 30 [class] cards; 6 do various stuff = 24 quests
- 50 gold: 9 win 3 with [class]; 15 play [x] [keyword] cards = 24 quests
- 60 gold: 9 win 5 with [class] or [class]; 9 play 50 [class] cards; 1 Tavern Brawl = 19 quests
- 80 gold: 1 play a friend = 1 quest
- 100 gold: 1 win 7 games, 1 play 75 Battlecry Minions, 1 play 75 Murlocs = 3 quests
- 1 pack (calculated as 100 gold): 1 spectate a friend = 1 quest
- Total: 72 quests
Now the Tavern Brawl quest only appears when Tavern Brawl is up, so from Wednesday to Monday (by rerolling at the right time, it's only truly unavailable on Tuesday), so when determining the available quests, take that into account.
So the best case, assuming one 50 gold quest in your quest log and 5 40 gold quests on cooldown, results in the following: 6 quests unavailable, 66 available. Total value: 19*40+23*50+19*60+1*80+4*100 = 3530, which gives us an expected value of nearly 53.5 gold for rerolling the quest.
Worst case, you have the 5 best quests on cooldown (all 100s and the 80) on a Tuesday, leaving you with 7 unavailable (65 available) and an expected value of just over 49 gold.
As you can see, even in the worst case (all 4 100s, the 80 and Tavern Brawl unavailable), you only lose 1 gold in expected value by rerolling the 50.
Here are the break even points depending on the number of quests on cooldown (completed in the last 3 days or currently in your quest log, not counting the one you're rerolling), assuming of course you only have 50s+ in your quest log:
- 0: Your expected value is necessarily 52.5 gold -> always reroll
- 1: Worst case, your expected value is 51.9 gold -> always reroll
- 2: Worst case, your expected value is 51.2 gold -> always reroll
- 3: Worst case, your expected value is 50.4 gold -> always reroll
- 4: You only lose gold in the worst case (all 4 100s on cooldown), even 3 100s and the 80 on cooldown breaks even at exactly 50 gold expected value -> reroll unless all 4 100s are on cooldown; no gain nor loss from rerolling if 3 100s and the 80 are on cooldown
- 5: Do not reroll if the total value of your quests on cooldown exceeds 430 (e.g. the 80, 3 100s and one 60). If your quests on cooldown total less than 430, always reroll. If they total exactly 430, no gain or loss, so whatever you feel like.
- 6: (only on Tuesdays if the Tavern Brawl quest is not naturally on cooldown) Do not reroll if the total value of your quests on cooldown (not including the Tavern Brawl quest) exceeds 420 (e.g. the 80, 3 100s and one 50). If your quests on cooldown total less than 420, always reroll. If they total exactly 420, no gain or loss, so whatever you feel like.
If it is a Tuesday or you're rerolling at the wrong time on a Monday or Wednesday (Tavern Brawl not currently available), only one case changes: If 4 quests are naturally on cooldown, you no longer break even if they are 3 100s and the 80.
Notably, this means that you always reroll if a maximum of one 100 quest is on cooldown. Even in the worst case (Tuesday, the 80 and 3 non-Tavern Brawl 60s on cooldown in addition to the one 100), you break even with this restriction.
While Tavern Brawl is available or the Tavern Brawl quest is naturally on cooldown, you always reroll if a maximum of 2 100 quests are on cooldown.
If 3 100s are on cooldown, but the 80 is not, you also always reroll.
If all 4 100s are on cooldown, never reroll a 50.
If a pack is worth less than 100 gold to you, adjust the value of the Spectate quest accordingly. You'll have to redo the calculations in that case.