From what I've seen and heard, generally magazines have relationships with certain suppliers/labels who send them samples of the clothes they're pushing that season, and/or the magazines call them and say they're doing, for example, a nautical shoot or a winter wonderland shoot.
I'm sure there are other commercial factors influencing all this at times but I'd suppose that a label would only pay if they had the shoot exclusively (ie. only using their clothes) - and the mag would label that as an advertorial.
However labels have to spend a fair amount on schmoozing the journos and stylists and hosting press events for them to establish and maintain those good relationships.
The cost of advertising varies hugely from publication to publication - call the sales team and ask for the rate card, which gives you standard prices. You can almost always bargain down from rate card prices though - read this feature for advice on that:
Hope this helps.