Nokia Lumia 710 - How much can Nokia Xpress reduce my data usage on my Nokia Lumia?

It depends on the web page. Usually Nokia Xpress can reduce your data usage by about two thirds, but it can be even up to 90 % per web page.

Your image settings also affect the data usage: the lower the image quality, the lower the data usage. You can change the image quality in the Nokia Xpress settings.