Hey, everyone.
After a minor foot surgery, I was exceptionally bored and playing TF2. After getting another item drop, I wondered how many of these I needed to get a key, and eventually, an unusual.
I did some math, here's the numbers ( They are calculated in USD, final totals will be converted to other currencies );
The important information is bolded, if you don't like reading.

If you could get 100 alternate accounts running ( AKA grinding bots ) on separate systems (items do not drop if the same computer has multiple instances of TF2 running), and not considering expenses to set up these separate systems you would still need to upgrade these accounts to premium in order for them to trade.

- Purchasing any item on the Mann Co. store will upgrade you to premium. The cheapest item on the in-game store is 50 cents USD 
( If you rent a common weapon, the Rocket Jumper for instance, you can then immediately purchase it for half-price. Normally, craft weapons are .99 USD, but after renting one you can buy it for .50 USD.) 100 accounts x $0.50 USD = $50 USD cost for making the accounts premium status.

- After tossing these alts onto a random item farming server, items will drop roughly once per hour. Assuming these bots will exclusively get item drops ( no paint drops or hat drops ), and assuming that they will get that drop once per hour exactly, the math turns to; 100 accounts x 1 item drop x 24 hours = 2400 item drops an hour x .5 (to convert them to scrap metal) =
1200 Scrap Metal a day. Assuming that a scrap metal equals $0.01 USD ( Due to TF2's economy right now it equals much less than that, but we'll use this number because it's easier to calculate ) That means that these alts will earn you approximately $12 USD a day. I am aware there is an item cap on how many you can find a day, but it will allow you to still find some certain items even past this limit.

- If you earn 1200 scrap a day, this totals to around 133 Refined a day. Divide this by 43-ish ( the current sell price on scrap.tf for keys and that totals to around 3 keys a day. 

- The math is really easy from here. If I wanted to pay off the original 50$ I spent in order to upgrade the alts to premium, that would take roughly 4 days. ( $50 spent / $12 gain per day, mentioned earlier = 4.16 days ) If I wanted to purchase a low tier unusual ( valued at around 10 keys ) it would take slightly more than 3 days. ( [3 key gain x 3.33 days] = {roughly} 10 keys )
Now, if I wanted to purchase a god-tier unusual ( We'll use a Burning Team Captain, valued at 3333 keys ), it would take a little more than 3 years. ( [3 key gain x 1111 days] = 3333 keys ) While this may seem to be a long time, a god-tier unusual is valued at around $6200 USD. That is approximately 124 times the amount it cost to set up 100 bots.


Whew.
Well, you made it. Here's your craft weapon.
Thanks for reading.