>I have been consulting on the side to a small software business
>who has asked me to develop a new version of their product.
>
>The client already has previous versions of the software sold
>to their clients, and if I were to take this project on, I
>have been offered a royalty on each copy sold.
>
>I'm wondering what everyone thinks of this. Has anyone done
>anything like this? Is there a typical percentage per copy?
>What are the benefits/pitfalls.
>
>I welcome insight into this.
>
>Thanks

This is very risky item
We do it and in some places we have success and in some paces we failed. It depend on agreement.
One of most protective way is your partners to not have access to the last version code and software install to require access key, which will be provided by you. In this case you will controll sales. Which is most problematic place - how many sales they made.

Standardly development cost in one well selling product is about 30%

HTH