It’s arguably the biggest challenge of web 2.0 – how do you start charging for something you’ve so far given away for free?The almost universal approach to date has been that you don’t; instead you find other revenue streams (usually advertising) that allow you to keep offering the core service without charging.After all, for social networks and free entertainment sites constantly courting the attention of promiscuous web customers used to getting what they want for free, suddenly charging would be seen as the ultimate turn-off.It’s a bit different for b-2-b though, where you inevitably expect to pay for any service with real value. That’s not to say, of course, that going from a free model to a paid-for one doesn’t hold its risks: you’re opening yourself up to a competitor offering what you do for free, for a start.It was interest then that I received the following email when placing a job ad on a site I’d used gratis for several years:“Matt, the position is now live on our site... It’s also worth letting you know that we have moved to a fee paying structure this year – due to increased traffic on the website and the time required to manage it. I’ve attached a rate card for you to see. Please note that we’re offering two free adverts before we start to charge, so do enjoy another free posting with us. On saying that, we continue to post all internship / work experience positions for free.”So I’ve now got to pay for a service I got for free. But you know what? Providing I still get value, I don’t really mind.The site, Gorkana, has earned my respect over the years by providing a great service for free. Its rates introduced also mean it's still more economical than competitors and, once compared to the cost of advertising in a national newspaper or with a recruitment agent, I’m still getting a good deal.Plus they’re softening the blow by communicating clearly and staggering the process with two more free placements. I.e. they’re continuing to demonstrate the quality of their service before asking me to pay for it.Time will tell if Gorkana’s got it right or wrong for charging, but for me it feels fair and surely that should the barometer for others to follow when it comes to pricing models?