You can save money if you buy the phone outright and use a prepaid plan instead of going on contract and getting the phone cheaper, but the prepaid plans will also have data -- you won't need to rely solely on WiFi.
I'm going to assume you are in the US... You can get AT&T Go prepaid for (from my understanding) about $50/month that gives both talk minutes and data.
If you can get an unlocked phone (or unlock the phone), you can move to Straight Talk (which uses AT&T's network, so same coverage) and get their unlimited plan for $45/month with unlimited minutes and about 2GB/month data before being throttled.
Also, T-Mobile has a $30 prepaid plan that gives up to 5GB data but only 100 minutes talk. If you go T-Mobile, however, you likely want to get a One S (the variation T-Mobile has) as the One X doesn't have the 1700MHz frequency that T-Mobile uses for 3G.