First off: How did you get so far into that minefield without stepping on a mine??

There is a table on page 372 of the CRB that lists the suggested starting pay, and the modifiers to that base pay depending on how hard or easy the run is. (it also lists the potential Karma awards on a different table on the same page if you are interested.).
Keep in mind, the issue of payment is a controversial one. There are many people, for a variety of reasons, that feel the listed payments are too small... And some who feel they are too big!
For what its worth, I don't use the table on page 372 at all.... (but this is because I have been GMing for years and have my own system for assigning payments for runs.)
If you are looking for a way to determine a run payment for your game, here are few things to consider:
1: How many runs a month are you expecting the Runners to do?
If you are expecting the Runners to be doing a run a week, then that would mean 4 payments a month... (or $12,000 before modifiers based on the table) If you are expecting a single run a month, then they are only making $3000...(before modifiers) Which can pose problems with just maintaining their gear! (lifestyle costs, used ammo and supplies, repairs/ etc)
2: Who are your characters? Are they street scum or High Rollers?
This matters a lot! NO ONE is offering $25,000 paydays to a bunch of nobodies. ON the other hand, a crew with as stellar reputation for perfect work, and a resume that spans years of action and hundreds of runs are not going to get out of bed for $500.
3: What does the run entail?
There is big different between "Rough up this salaryman and put the fear of God into him" and "break into the Neo-Net Zed zone and bring back the one of kind McGuffin that is worn around the CEO's neck" The more planning, the more danger, the more heat a run can generate, the more the pay should reflect this... (NOTE: this is the base run... Character actions can always change the difficulty of a mission... If the players were to sneak in and get an item, but their idea of "sneak" is using an ATGM to blow open the doors and then run in with miniguns.... well no bonus pay for the heat they attracted!)
4: Who is paying for the run?
Not all jobs come from Johnsons with multi-billion dollar expense accounts. Some Johnsons are just regular people who need something done that is illegal (or at least immoral, or just "Grey").. A Johnson that represents a low income Arkoblock trying to get the Mob to leave them alone can't pay as much as a Johnson from Renraku. Also keep in mind that payments don't have to be in cash... They can be in just about anything... from stocks and shares to fresh fruit! Course, the runners would like cash... but that is just not an option all the time. (My character's crew got paid in Coffee beans once!)
5: What is the theme and atmosphere of your game?
This matters more then people think, as payments can throw the theme and mood of your campaign right out the window. IF you are trying to portray the world as a dark and gloomy dystopian where life is cheap and living is hard, handing out multi thousand dollar payments really works against the tone you've set.
6: how fast do you want your character to progress?
Money is Power in Shadowrun... Both thematically, and in the crunch of the game. When the money flows like water, so too does the reagents, foci, ammo, guns and ware... which can lead to a dramatic increase in the player power level relative to their karma and reputation. Which can lead to game imbalances right quickly (If you one of those that subscribe to "balance in all things" game philosophy) On the other side, if payments are not high enough, then the character start going backwards! If they can not cover their costs (assuming they are not morons and overspending) they can not really progress.
It can take a while as a GM to get a handle on payments, especially for a new GM. And the "correct" amount to pay is going to change just about every campaign and group of characters (as all campaigns and characters have different costs). My advice to you would be to start on the low end of payments and then slowly increase them as you become more comfortable. After all, its easier to slowly increase rewards to compensate for UNDER payments in the past than it is to compensate for OVER payments that could get you into trouble later on...