Thomas Fuchs has written an article on creating your own Ajax effects.

The article goes beyond the builtin effects that Script.aculo.us has, and shows you how to create your own. The example used is a cash register effect that counts between amounts as the register changes.

The end result is a nice easy effect that you can use:

new Effect.CashRegister('total',150,{duration:10});

Then you can explore callbacks to do further processing, transitions, effect queues, and more.