I agree largely with Ted and Pig, although I don't think you're always going to get face value, just a reasonable price, depending on the matchup. I've never been turned away from a match, and the most I ever paid was 100 quid for ManU v. Arsenal a couple year's ago (right before their FA Cup matchup the following week.) I determine what the match is worth to me, and that's what I expect to get. That match was so tight, touts were asking 4-500 quid, and of 5 or 6 I saw, only 1 had tix in hand! I nearly got a bloke to sell me his mate's at face, but the guy showed up at the last minute. I was about to give up as the match was starting when I decided to walk up to a ticket window (the ones at Highbury that have gates) and asked in my obviously-American accent, "how much would it cost someone who flew 8000 miles to go stand in the corner." He replied, "I'm afraid I can't let you do that, mate. But he's got a ticket if you want it." His co-worker at the next window sold me one he had for 100, which was my max.
Sometimes you gotta get creative, but if you really want, there's always a way in. The problem is, the touts have the obvious upper hand in these scenarios, because we, as fans, give it to them, with our frenzied passion. That's why you should treat it like gambling. Determine how much you're going to spend
before you get to the venue, so you don't let your adrenaline once you're there determine your max value. If you don't get it, be prepared to walk away.