When is Future's Market Efficient? (My attempt to answer)
Earlier I made a post asking about this and I basically got a response that said, ... "It isn't" (to be fair, they said more than that)
So, I'm going to try and give a little more of my own logic behind this rune and possibly answer the question for myself, which I was just hoping to be lazy and have someone else do for me. It is also very likely that my logic is in some ways flawed and missing some important factors. I also want to apologize because this is likely to be a very long post, .... and 99% of the people that click on this topic will probably not even read this far after having glanced at the overall size of my rambling.
First, I want to address the "It isn't" response.
Well, it is not a keystone, .. so it isn't likely to be a game-breaking rune that gives you an extra 1000 gold worth of value. I see it as something that is mostly beneficial for champions who can gain an early advantage and snowball off of it, giving the rune more continued value as the game goes on.
Mathematically though, ..
I'm going to start by assuming that you intend to get the maximum value every 5 minutes in a match. For simplicity, I am going to assume that gold generation (20.4g per 10 seconds) starts at 0 seconds instead of 1:05, giving an average of 144.4 gold per minute Your average cs is 7 minions per minute as a mix of melee and caster minions, averaging 17.5 gold per minion and totaling 144.5 gold per minute. Using these two, you earn 245 gold per minute. Again, for simplicity, you have no other form of gold generation; no champion takedowns, no gold items, no Kleptomancy, no champion abilities, no other objectives taken. ...
Future's Market grants 150g + 5g per minute. @5:00 = 175g @10:00 = 200g @15:00 = 225g @20:00 = 250g
So at 5 minutes, when you return to base you will have 1225g in your pocket, and an additional 175g available to spend, totaling to 1400g
Upon leaving the fountain you will be in the hole by 225g.
At 10 minutes you will have accumulated another 1225g, BUT subtracting the 225 from before you will have 1000g in pocket with FM providing 200g available to spend, totaling 1200g.
Upon leaving you will have -250g.
However at this point you would have normally accumulated a total of 2450g, but you have spent a total of 2600g
At 15 minutes, ... 975g in pocket + 225g from FM = 1100g to spend
Upon leaving you will have -275g
However at this point you would have normally accumulated a total of 3675g, but you have spent a total of 3700g
At 20 minutes, .. 950g in pocket + 250 from FM = 1000g to spend
Upon leaving you will have -300g
However at this point you would have normally accumulated a total of 4900g, and now you have spent 4700g
I think we have some useful in formation here now.
There is clearly some diminishing return on your investment if you use this rune to it's maximum extent consistently, which is actually what I expected, but I was waning to see it, .. and maybe this will help someone else understand the way this works as well.
I also want to point out that you can go into less than your maximum available debt and delay the point at which the diminishing return catches up with you. If you go more than 10 minutes without using this feature, you offset the -50 gold fee.
As I said before, I believe the distinguishing difference in if / how well / how quickly you are able to recover this diminishing return is if you are able to use the extra gold you acquire at an earlier point to snowball. If that extra power helps you get first blood or first tower, .. if it helps you get a shut down, or improves your ability to wave clear more consistently, roam more quickly and effectively, take other objectives and buffs, ... this rune starts to become more and more valuable.
This rune is not likely to save you if you are falling behind. In fact, .. at that point you might actually be better off not using it at all.