Doing what though? Several hours spent running commodities for the CG netted me less than ten, normal trading even less (which really makes me wonder due to you stating you earned over a 100 doing similar), mining, core and traditional for a few hours netted me similar, FSS scanning a system fully of signal sources and astrological bodies netted me zero, turning in the data, 1, mission running for a few hours, less than ten again, massacre mission for 30 pirates, around an hour spent and 97. I was under the impression that nobody needed to change the way they played to earn arx, but let's face it, if you want to earn any amount of them combat is certainly better by far than other core activities.
I could easily earn the cap of 400 in a week if I spent my time killing things, anything else and I get nowhere near it, lucky if I get even a quarter.
Strange, I earned around 90 Arx yesterday just doing a few delivery and fetch missions - also made around 30m credits in the process.