How difficult is it to make an accurate comparison of the cost of a mobile phone package? Why do you think this is so?

Visit www.vodafone.co.uk, www.o2.co.uk, www.orange.co.uk and www.easymobile.co.uk and compare the prices these companies charge for their products. How difficult is it to make an accurate comparison of the cost of a mobile phone package? Why do you think this is so?