I've just changed the belts on my ST. They cost about £32 apiece from John Hacketts who will post them out for next day delivery.
Fitting them is really easy. All you do is take the belt covers off, get the bike on a paddock stand so you can turn the back wheel by hand, whip the plugs out so that there is no compression and put the bike in gear. Then you just turn the back wheel by hand until the timing mark on the crank pulley lines up with the mark on the engine casing - the other timing marks on the camshaft pulleys should line up with their respective marks on the casings and the horizontal cylinder will then be at TDC. Then slacken both the bolts on the adjuster pulley and take the old belts off.
Feed the new belts on and the only mildly fiddly bit is fitting the vertical belt. It has a helper spring on the camshaft so that the pulley will want to flick up or down away from it's timing mark, so you need to make sure that you hold it in line with the mark while you put the belt on. Keep holding the marks in alignment while you roughly tension the belt and just nip the adjuster bolts up. Repeat for the horizontal belt which is easy because the timing marks stay put.
Now the new belts are on, start with the horizontal cylinder and squeeze the adjuster to take up the slack until a 5mm allen key will snick between the belt and the fixed idler pulley reasonably easily but a 6mm allen key will be a struggle, then torque both the adjuster bolts on that adjuster to 25nm (I think from memory) you might have to repeat this process a couple of times until you are happy that the allen key test is repeatable once everything is torqued down.
Once you've done the horizontal cylinder, turn the engine over with the back wheel again through 90 degrees so that the vertical pot is now at TDC and repeat the adjustment for the vertical belt - it's a good idea to leave a little more slack in the vertical belt because the back pot runs hotter and you can allow for a little more expansion so I make sure a 6mm allen key will snick through and a 7mm wont - it's better to run the belts a tad slack than too tight.
Torque it all up then turn the engine over a few times to make sure that nothing is fouling and pop the covers back on.
It's easier to do than describe and there's a good video of it here
http://www.youtube.com/watch?v=9vzPZ84ZRjU