You can certainly use a Slingbox with your limited data plan, whether you want to use SlingPlayer on your smartphone, Watch on Slingbox.com with your home computer using your home Internet provider, or Watch on Slingbox.com while away from home using another Internet provider (such as hotel Wi-Fi or an Internet cafe).
But you'll need to watch your data usage very closely to make sure you don't exceed your limit. Doing that can be costly.
Check with the Internet provider with whom you have the limited plan
(usually either your home Internet provider or your mobile phone carrier). Most providers offer online (or mobile phone-based) tools that you can use to check your data usage on a regular (even daily) basis. Your actual data usage can vary dramatically based on connection speed, Slingbox used, available network bandwidth, connection quality, and many other factors. The best way to monitor data usage is by using the tools that your Internet provider or mobile carrier offers you.
Does SlingPlayer or Watch on Slingbox.com offer a way to track usage?
No, neither product offers this feature. Again, the best way to monitor usage is to use tools that are provided either with your smartphone, or by your Internet provider.
However, SlingPlayer for iPhone, SlingPlayer for Android Phones, and SlingPlayer for iPad do offer a way you can slow down network usage while you're watching your TV with SlingPlayer.
With any of these three products, you can go to Settings and choose to have SlingPlayer limit the bandwidth that it uses to no more than 1,024 Kbps at any time. While this won't prevent you from going over a particular limit, it does slow down the rate of network usage, which means that you'll use less network bandwidth during the time that you use SlingPlayer. However, doing this may affect video quality under some conditions.