You do not have to spend a lot of money for a UML diagram designing software because Umlpad among other free tools exist. Umlpad supports the inclusion of multiple UML diagrams that can be modified as you please.
The diagrams can be worked on as you would with any expensive editor. One major difference between Umlpad and those expensive editors is that Umlpad is not cluttered with features. Use Case and Sequence diagrams among others can be added.
For your convenience, Umlpad will allow the printing and exporting of the sequences you have created. This is a standalone application that is not able to integrate with any other UML tool. Umlpad is an open source editor therefore its source code can be downloaded and modified as you wish.