I had two fairly effective ways of dealing with this issue.
The first was an early on run. The job was to break into a minor facility and make minor changes to personnel data and obtain security codes. The personnel data was on hard copy as it hadn't been added to the server yet, and the codes were on the building's node. Involving hacker and team, yay.
Anyhow, run goes nice and smooth, they evade the relatively lax patrol, get the data, drop off the modified file into the personnel file cabnet to be added to the trix. Well, the place was a manufacturing plant and one of the players had the idea, "Hey, since this is basicly a small factory, they have to have a small nurse's station or something like that. Med supplies and drugs would sell easily.
So, I let him find it and sure enough, some painkillers, slap patches, medkits, a few of the less illegal mind enhancers that seemed appropriate, all inside a simple glass paneled cabinet. Locked. So the player decides to put some of his clothes against the glass to muffle the noise and break the glass. Works fine. He gets stuffs his pockets and meets up with the others and they vacate.
At the meeting the Johnson is all smiles while they sit at the table in the back room of the bar.
'The codes?"
"Right here." The face passes the datachip.
"So how did the run go?"
"No problems whatsoever."
"Oh good, because when I saw this, I got worried." The Johnson turns on the vidscreen behind him and a news story about a break-in at their facility was on the news. The story said that it looked like it was junkies and nothing else was taken.
"Hey, no big deal, they have no idea this was a run. They said junkies."
"No big deal... NO BIG DEAL!?" the Johnson get a furious look in his face. "Standard procedure after a break in is to CHANGE ALL THE CODES YOU IMBECILES! THIS IS AS USELESS TO ME NOW AS YOU ARE!!" And with that he snaps the datachip in his fingers. "I'll give you one more chance to get the codes and save your reputation and paycheck, at a reduction. But it has to be done tonight, and it has to be done right this time."
So the runners found themselves doing the same run again. Only this time for less pay and with the guards on full alert because they just got chewed out by their bosses for negligence.
The second thing I do to keep this in check is I always ask who or how they intend to sell the info. If they don't have options in who to sell to, the contact typically will figure it out and cut the deal way in their favor. "Don't like my price, fine. Who else you going to sell data that hot to? And in a week that paydata will be worth half what it is now, time is money when it's data. So go ahead, find someone else if you don't like my offer."
I once had them try to auction paydata on the matrix. Oh, it seemed to go real well at first. They provided credible evidence they had the real thing and everything. Course, as soon as the bids hit that magic number, the one where it costs just as much to hire other runners, the bidding suddenly stopped. Then the jamming and the bullets started soon after.
And for the really minor stuff I don't care much. Every now and then I'll throw in a commlink that has advanced modifications to make it self-destruct or shock the person who tries to use it without the right access or something, but I'll usually just let them have the standard 30% fenced gear amount. It usually doesn't seem too worth it when they realize they get less then a third of it's value.