Shelly - Nexus 4 for Sprint unlikely - This article explains why
http://www.theverge.com/2012/10/29/3569688/why-nexus-4-does-not-have-4g-lte
Roaming Data In Europe varies (it's a competitive, somewhat regulated market that varies by location, Europe is not one country ).
Speaking from personal experience, in the UK you could get PAYG (Pay as you go - thats prepaid to you US folks) , One month unlimited HSPA+ with "Three" for 15GBP= 25USD( at current FX rates)
Other carriers charge more .. But in general, local data is cheaper than US roaming data - e.g. Some US friends of ours were complaining that AT&T had billed them for $60 for the previous days data in France - good job it was a business expense.
Unfortunately you need to do research, and think and stuff, if you want to avoid throwing money at your carrier in order to avoid having to think
If you are visiting somewhere, ask the locals what they use, in advance. If they are thrifty, you're likely to get good advice.