I have looked at it personnally and used Excel to find the formula. It's quite complicated but once you understand it, it's easy to know what worth more than other boosts.
(Original Time) x (Factor A) x (Factor B) x (Factor C) x (Factor D) = Time with all boosts taken in consideration
So yes, there is in fact 4 categories of boosts.... even if Plarium count them all in the same box!
How to find FACTOR A, B, C and D:
First we must divide the boosts in 4 categories
Category A: Repute
Category B: Daily boosts, KVK and Fury winning bonus
Category C: Town skins and time boosts
Category D: Knowledge, Buildings, SH, Aesir, Hero skills, Chaman skills, equipment...
All boosts in all categories are added to get a total for each category. Let's say you have a beast town skin (10% bonus) and a 30% bonus for 30 minutes, you will have 40% of bonus in Category C. Once you added the bonuses, you can find the Factor C with this formula: 1 - 40% = 0.60
You can make this for Factor A, B and C.
For Factor D, the total will be passed 100%, so it's obvious you won't have 100% less time like it happen with food consumption. So you add all the bonuses and you will than find the factor with this formula:
1 - TOTAL Category D DIVIDED BY (TOTAL Category D + 1)
This gives this kind of factor
100% bonus = 0.50
200% bonus = 0.33
300% bonus = 0.25
400% bonus = 0.20
and so on...
When you will understand the formula, you will realise that winning a KVK (10% bonus) will have much more impact than getting the 100% bonus from new equipment... Open Excel, try it and think about it!