The skin you get from Trade Up Contract is randomized. The wear, however, is not. There is a specific formula used to calculate the exact float value of a resulting skin, which will be shown and broken down below.

First, it is important to know that skin wear is more than just text. There is a hidden float value (0.0 to 1.0) that determines exactly how scratched or dirty the skin is. While 2 skins can both be Minimal Wear (0.07 to 0.15), one could be almost Factory New (0.074), and other almost Field-Tested (0.14).

Here is how the skin wear is split (click to enlarge):



A lot of skins do not utilize the whole range and have their own minimum and maximum. Such example is AK-47 | Black Laminate, with a minimum of 0.06 and max of 0.80. If you want to find min/max for any skin in existence, I strongly recommend CSGOStash[csgostash.com].

Here is a breakdown of the formula:
(X)*(max-min)+Z = W
X - Average float value of 10 input skins
max-min - Float value range of the skin you want
Z - Minimal allowed float value of the skin you want
W - The exact float value the skin you get will have.

Example:
Wanted skin: AK-47 | Black Laminate (Factory New)
Skins used: 10x PP-Bizon | Carbon Fiber (Factory New), each with float value of 0.01 for simplicity

X = 0.01
max-min = 0.80-0.06 = 0.74
Z = 0.06

0.01*(0.74)+0.06 = 0.0674 (Factory New)

Once you know what skin and wear you are looking for, you can reverse engineer the formula by starting with the desired result wear. You will get the average float value of input skins required.

AK-47 | Black Laminate (Factory New), float value of ~0.063

X*(0.74)+0.06 = 0.063
X is 0.004, so you will need 10x FN PP-Bizon | Carbon Fiber's with an average float of 0.004.

You can also play around with CSGOFloat Trade Up Calculator[csgofloat.com] to see what float values give what results. The website offers a comprehensive database of skins, allowing you to look for potential trade up ingredients.

Steam Market can also provide necessary skins, but inspecting every item by hand is tedious. There are Chrome browser extensions, such as Steam Inventory Helper or CSGOFloat which can make the job easier.

Would you prefer to watch a video than read a wall of text? Well, then you can do that too!
https://www.youtube.com/watch?v=1I-hwnrEMKA - TrilluXe's video

Credits to /u/lurker111111 for finding this formula and sharing it with the community.
http://www.reddit.com/r/GlobalOffensiveTrade/comments/3a57au/psa_float_values_and_tradeups/