Well the Marin reforms actually had the state pay for the equipment where as before, it was down to the man himself to pay for his own. There is much more to it of course, but it does make some sense that upkeep could be large. However booty and loot was more often or not how a legionare got paid rather then salary, which was more food stuffs and salt, not just money.

Indeed, to be able to mainain and upkeep those imperial legions Rome had to draw on a vast economy in marian and later times. Perhaps by the time you develop your techs and conquer half the europe you'll be able to field those legions. :)

Weren't the equipment costs simply taken out of the men's pay. Sure it was a large initial investment for them. But it pays off in the end and a lot of their equipment could actually outlast the service time of many solders.