Shadowrunner pay can vary a lot, from hobos with shotguns gunning down a suit for beer money, to pros getting paid 5 or 6 digits to get a bleeding-edge prototype out of a secure research facility. From a fluff perspective, shadowrunners who are pros should get decent pay, otherwise there are a million other well-paying and less dangerous jobs they could be doing. From a metagame perspective, you need to give the group enough money that the characters who need money for advancement (street samurai, riggers, and deckers) can keep up with the awakened characters, who are more dependent on Karma for advancement. Runner pay can vary because character creation lets you make characters of widely varying ability. A cohesive group of specialists will pull in more money than a group of more street-level characters, or powerful but messed-up characters.
There are lots of ways that shadowrunners can fall through the cracks into the shadows. They can get burned out or otherwise discarded from the megacorps, or have their eyes opened and leave in disgust on their own. They can be street punks working their way up the criminal food chain. They can be bored or jaded people looking for excitement. They can be idealists and windmill-tilters, ranging from men and women of strong morals and honor, to deranged whackos. There are even characters literally born into shadowrunning, following the footsteps of their parents.
The public view of shadowrunners will not be that different from the view people have of criminals today - they will be scapegoated, despised, and at the same time romanticized. Keep in mind that the corporations not only control the culture, but, like companies today, have heavily infiltrated the counterculture. The same megacorp that decries shadowunners in an op-ed piece will also heavily advertise their new "Lars, Samurai for Hire" trid series, and might quietly hire a group of shadowrunners to sabotage a rival company's factory. Note that since terms like "shadowrunner" and "street samurai" have been so romanticized, there will be a lot of posers calling themselves shadowrunners or street samurai.
The average person's life is generally worse off, despite all of the new tech and toys out there. First of all, the wealth demographics have shifted to be more in line with a third-world country. There is a large underclass of people with no SINs and no rights to speak of, living in crumbling slums, getting by with scrounging, criminal activities, and horrible jobs (working at "toxic castle" factories, bunraku puppets, etc.). Wage slaves have it a bit better, but they still work long hours with no union, no health insurance, no overtime, and lots of oppressive bureaucracy and petty little rules. They do at least have regular food to eat (even if it is mostly flavored soy or krill) and a place to sleep. They de-stress by hitting the nightclubs and going wild (just like a lot of Japanese salarymen go out drinking), or immerse themselves in VR, escaping to a fantasy life for a while.
The corporations punish disloyalty, and will probably be quick to get rid of an average wage slave who dares to complain, whether it be about impossible deadlines, sexual advances from a boss, or moral qualms about some vile thing or other that the company is doing. The only upside for wage slave is that they will rarely be worth the effort to "disappear" - instead, they will be unceremoniously tossed to the curb. More valued employees such as company men or researchers have more perks, but this comes with the price that it is much harder for them to extricate themselves from the company. The company does hire people with a bit of a maverick side - mainly hacker, riggers, and wage mages. Typically, they will be given enough slack for minor mischief that they won't chafe at their leash.
Using all of the rules in the book can make it all but impossible for the core concept of the game, shadowrunners, to be possible. Play up that data is balkanized and treated as a commodity rather than something freely shared, that there is a huge glut of data, and that it is very unreliable since it is so easy to falsify, or hack in and erase. Also, shadowrunners are more or less an accepted way of doing business, and the occasional stolen data or dead security guards are part of the cost of doing business. Corporations will tend to prioritize getting their property back, or finding out who hired the runners, over catching the runners, so they usually won't have to worry too much about pursuit after making the hand-off. This goes out the window if the group violates the unwritten rules (too much collateral damage or civilian casualties), and there might be individuals within the corporation who take things more personally.
Corporations generally wield more power than nation-states, because just like today, they have a huge amount of influence over them. National governments are smaller and weaker than they are currently. Despite how things like the military have been scaled back, though, nations still have actual armies, rather than security squads, with things like battleships and fighter jets. They will typically use these resources at the behest of a corporation, though, rather than acting against one. There have been a few attempts to nationalize corporations in the Shadowrun universe, and they have typically eventually crumbled under the collective pressure of the megas.
Shadowrunners typically depend on a network of contacts and illicit businesses or organizations to get gear, jobs, fake ID's, places to lie low, and otherwise to simply function in the shadows. It is one of the reasons why contacts and reputation are both so important.
Megacorps dominate the major markets, and let smaller companies fight over the scraps. I think it is a given that a company that gets too successful, or that invents something new and useful, will be quickly bought out, with the few that balk at this being targeted for a shadowrun. There is also a fairly big grey and black market, but the corporations have probably even infiltrated them to an extent.
I am not sure that Shadowrun has an intended setting and tone. Not a singular one, anyway. One of the major strengths of the Shadowrun franchise is that it has been capable of supporting a wide variety of playstyles. But that is why it is a good idea to sit down with your players and decide what kind of Shadowrun game you want to play.
