I've been using the dry ice on a tank with a dent the size of a baseball. I now have it down to the size of a quarter and am still working on it.
Get the dry ice from your local grocery store for free (don't forget to take a cooler with you). They usually get it certain days of the week with their frozen deliverirs and just put it outside in the sun to disappear. Since mine was so big I have spent several days and hours on it due to it's location (several inches forward and to the left of the fuel cap on the curvature of the tank).
It will work but it usually takes many attempts ($ cost is nothing--just some of your time). Certain "light" dents will usually pop right out with one or two attempts, but these are usaully tiny hail damage like dents. Location of the dent also is a very significant factor as to how many attempts or "if" it will even pop out.
You need to hold the ice against the dent for several minutes. USE GLOVES!!!
You may want some ear plugs also, when the metal is very hot from sitting in the sun and you hold the ice over the dent it tends to "scream" very loudly. Make sure the ice is at least an inch larger than the diameter of the dent. You can use light pressure and move the ice very slowly around the area of the dent to widen it's effect. When the entire dent and an inch or so around the dent develop a light but visible "frost" remove the ice and wait. If it does not remove the dent keep trying it. My work in progress has been yeilding results but very slowly. The dent only comes up a little at a time, hardly visible at all. Very hot sunny days also help, make sure the dented side of the bike is facing the sun directly. Hope this helps.