Here are a few ways to save your Monthy alloted Bandwidth:
Using local caching, such as memcache, can greatly reduce the actual requests to make to our solr servers, thus saving you bandwidth
Since the allocated Bandwidth is PER INDEX, you could setup Solr Replication and setup your local application to perform round-robin requests in all of your replicas. This way, the bandwidth is saved by ballancing it between the ressources of multiple indexes. For example, if your account has 1 Gb PER INDEX, you will create Index A and replicate it onto Index B, and make requests in both of those in a round-robin fashion, thus gaining 2 Gb total of bandwidth for your index.
Make sure your solr queries will return as little data as possible. For example use the rows or fl parameters for the solr /select requests, to only return the records and the fields you really need. Any other data the gets returned will be counted as extra bandwidth.