Meaning of 一人付一半 in 车费我们俩一人付一半，怎么样？
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.