It costs me about $30 every time I fill mine. Sometimes if I get the right guy (I go to AirGas), he doesn't charge me. He claims they lose so much when they fill the tanks, it's no big deal not to charge me

You should be able to have it filled at any welding supply store, just make sure to find one that has already been filling tanks for other people. I checked at one store and they said they would not do it because the one time they did the person had their tank fail right afterwards and lost all of their semen.
The tank failing is probably the biggest con to AI. You can't predict when it will happen, or if it will happen. Even with a brand new tank, it could be defective and fail on you. Most manufacturers have a warranty on the tank, but they do not compensate for lost semen (when you lose it that $10 a straw semen turns into $100 per straw semen

) Buying a used tank can be risky, never get one that has been allowed to go dry, that is supposed to damage the insulation in the tank and make it not hold semen. There are certain surfaces you aren't supposed to set a tank on, I can't remember exactly which ones (metal I think is one) so I keep mine on a piece of cardboard just to be safe. You also do not want to store the tank in a living area/room. If it is leaking it can cause asphyxiation. Nitrogen is also extremely cold ( -320 fahrenheit if I remember right) so you do need to be careful not to ever get any of it on you as it will cause instant freeze burn. You can pick up the canes with your fingers, put you don't want to hold them very long as they are very cold. I keep my tank in the house, but it is in an area that is between the living room and the kitchen and near the front door, so people aren't constantly in the same room with it.
I do know a long time Saanen breeder who had one of his tanks fail while he was at Nationals one year. He said the tank was completely full, most of the semen was from purebred Saanen bucks who were long dead and he was the only one who had the semen from them so it was definitely irreplaceable. He also had most of it sold, pending shipment. He valued the semen in the tank at $10,000 so it was a huge loss for him. There are some places around that will store semen for you, I'm thinking they are insured so if something happens while it is there you get compensated for it. I do know people who store their excess semen at these places, partly so they have more room in their tank and also so that in the event their tank does fail they still have some semen.