@sensorcheck - Others I'm sure will chime in here but I think your options are Base64 or Base85 if you are transferring binary/numeric data. I personally use Base64 to convert a byte array to a string before publishing. It works well. It's on my list to also look at Base85 to further compress it but isn't a huge priority right now. For me, the extra data operations isn't worth the headache yet to convert to Base85. Here is a good reference post asking a similar question that links to several examples with libraries for Base64 and Base85.
I personally use this: Base64RK
From my experience, you rarely hit the data (MBs/month) limit but rather you may hit the number of data operations/month. So the name of the game is to squeeze as much data into a single publish event. So on a related topic, I also use the technique to assemble a JSON Array of data to reduce data operations. This is explained here. Each member of the array is a new reading from a different period of time. I only publish once the max number of characters would be exceeded OR every 20 minutes whichever comes first. That works for my use case. It may or may not depending on how real time the data needs to be.