Always buy the first 3 that cost 10, 50, and 250 fame points respectively. They give +20% to fame points earned after each battle (5+5+10) effectively making multiplier to fame points become 1.2 instead of 1.
Bonus for 1000 (gives +20%, making the total become 40% ie 1.4 multiplier):
Take 1,000 if you are going to earn at least 7,000 fame points after buying it.
Proof: let x be the amount of fame points you will need to earn after buying the said multiplier to pay it off
x - x * 1.2/1.4 = 1000 x * (1-1.2/1.4) = 1000 x=1000/[(1-1.2/1.4)] x=7000
In this solution, we subtract base value * 1.4 multiplier (which is x in the equation) from base value * 1.2 (which is x * 1.2/1.4 in the equation) to get 1000 points that are spent on bonus. Basically we are comparing 1.4 multiplier with 1.2 one and seeing when 1.4 pays off.
In reverse: 7000*1.2/1.4=6000 which is 1000 larger than 7000 and we have paid our "debt". After that it's all profit.
Bonus for 2500 (gives additional +20% for a total of +60% ie 1.6 multiplier):
Take 2,500 if you are going to earn at least 20,000 fame points after buying it.
x - x * 1.4/1.6 = 2500 x * (1-1.4/1.6) = 2500 x=2500/[(1-1.4/1.6)] x=20000
Bonus for 5000 (gives additional +30% for a total of +90% ie 1.9 multiplier):
Take it if you are going to earn at least 31,666 fame points after buying it.
Same procedure applies as proof just switch 1.4/1.6 from last example with 1.6/1.9. It's previous multiplier (in this case 1.6) divided by new multiplier (1.9). I hope you get the idea how it works.
Last 4 bonuses: you can calculate it yourself 😉 but tbh, if you are planning on using them, you probably already know all this. This guide is primarily meant for new players to clan wars or players who are struggling to figure out how all of this multiplier stuff works.
As for the tank, it will most likely cost around 27-30k fame points so brace yourself. You can check my article on previous clan wars campaign here:
Also one tip for new players, don't be sad if at the beginning of the campaign you don't earn enough fame points to get in top 11500. What matters is that you play every evening possible and especially towards the end of the event (last week or so) when most of the battles will be fought with x5 multiplier (on top of your personal ones), if your clan manages to snatch some provinces to make that happen of course.