How can I create a lighter & darker version of "Pantone Uncoated Violet U" to keep printing cost down?

Hi,

I need to have a business card printed and I am not familiar with the printing process. The design contains black and three different purples. In order to keep the printing cost down I will need to reassign all objects to the same PANTONE color within Illustrator.  If I were to use Pantone Uncoated Violet U, how would I create a lighter and darker version based on this swatch?  To reiterate, the result has to be two color printing (black and one purple and not black and three purples). 

Any assistance is greatly appreciated.

Thank you!