Xavier Niel's Free Mobile offers a 100-gigabyte cap on their 20 euro service, which Jennie bought in Paris. For 20 euros, Telecom Italia offers 5-10 gigabytes. With Free soon to turn on Italy's fourth network, competitor Fastweb brought down prices to compete. In Canada, all three carriers brought prices down before the fourth entered so I expect TI and Vodafone to also adjust prices.
Xavi changed the European Internet when he offered triple play in France for 30 euros. Germany's regulator, Mathias Kurth, told me he pressed his companies to come closer to France's price. The lower mobile prices are slowly spreading to other countries. T-Mobile Netherlands recently went "unlimited" although DT hasn't done that in that in Germany, which recently went from 4 to 3 carriers.
The anecdotal evidence is strong that prices move 10-20% when a market goes from 4-3 or 3-4 primary carriers.