The price of wristbands has been rising steadily the past few years. What was once $25 has become, last year, $150. After the lowest price level bands are sold (usually about 2,000 of them), they raise the price for a second batch. Bands sold at the conference itself could be even higher.
Wristbands are sold in three ways. Mid-February, the bulk of them are put on sale at Waterloo Records. There has recently been a lot of secrecy about when they will go on sale, with SXSW offering to notify those who put their Austin-only cell phone numbers on a list for the purpose. While the sale of wristbands used to last for days or weeks (if they sold out at all), they now tend to go all in one day, with long lines at Waterloo.
Another way to buy wristbands is through a badgeholder. Every badgeholder gets the option of buying one wristband at a price lower than the max.
TIP: Those without wristbands can go down to the convention center and ask those waiting in line to pick up their badge if they have committed their wristband to anyone else. Most people aren't even aware they have the right to buy the band, so this is a good way to latch onto one.
Finally, after everyone wails and moans about how hard they are to get, SXSW usually puts more on sale the week of SXSW.