Meaning of 一人付一半 in 车费我们俩一人付一半，怎么样？ In this sentence:


  车费我们俩一人付一半，怎么样？


I don't fully understand the meaning of 一人付一半. 

I have found that 付一半 can mean "pay half", but 一人 confuses me (if it refers to one person, why is that form used instead of 一个人?).
一人 can be thought of as 'per person' or 'each person'. 

This grammatical construction is extremely common in Chinese. I think it's called topical construction (correct me if I am wrong here). Essentially, you have the topic of the sentence (bus/taxi fare) at the start, followed by the subject, verb and the rest of the object. 

So let's break it down:


Topic: 车费 (bus/taxi fare - need more context to determine the exact
nature)
Subject: 我们俩 (We [two])
Verb: 付 (to pay)
Object: 一半(车费) (half [of the fare])
Adverbial: 一(个)人 (each person, per person - alternative wording: 每(个)人)


Possible translations:


We split the fare by paying half of it each.
We both pay half the fare.
Each of us pays for half the fare.


I missed the last bit of your question the first time around. The measure word 个 is omitted. It means the same whether it's in there or not (in this case). However, 一人 in this sense is also used in some set phrases (such as 一人做事一人当 - one is responsible for one's own actions), and in those cases, you cannot use 一个人 in its stead. Fivesheep's answer has enough examples to give you a good idea how often it's used.