CAL help

I need help. We are currently paying £400 ex vat for 5 SBS cal. These allow us to use the server and exchange.
We are running Windows small business server V2003.
Have just looked around and I want user cals rather than device ones.
But the price of these varys widly, first I looked on dell
Dell
Then amazon
Amazon
And finally just a shop I found
genshop
Why are these prices so different and am I missing something, I just want my users to be able to log on and pick up there e-mail via exchange.