If you send a few SMS each hour, the SMS Server Tools are the best solution for you to build a very stable SMS Gateway.
But if you send thousands of SMS each hour you should ask your phone network provider for a SMSC Large Account. It may be much cheaper for you and also faster. On http://www.bai.de you can buy software for Large Accounts. The SMS Server Tools software does not currently support them.
When you send a message with this software, you have to pay exactly the same price as when you send normally with your mobile. Most phone network provider take a fixed price per SMS and also offer tarrif options to lower the price. You can save a lot of money by selecting the right tarrif for your SIM cards.
Performance note: Sending a message takes 5-10 seconds. If you need a faster solution you can attach more modems to your computer. There are multi-serial Interface cards and also external seral boxes with ethernet connector available. This software supports up to 64 modems on one single computer, this limit is easily changeable.