A legionnaire would earn 450 sesterces per year in the late republic to 900 sesterces under Julius Caesar and Augustus to 3,600 sesterces in the 3rd century CE. 40 to 50% of this was withheld from the legionnaire to pay for food, equipment and clothing.

Now I'm going to use the 3rd century CE to do the comparison to today's money but one thing to keep in mind is money had very different value. We can sometimes know how much something cost at a certain place and time but goods varied more over distances as well as the fact that supply and demand functioned differently than today, so its a VERY imperfect measurement.

So a legionnaire is costing the government 3,600 sesterces a year. What does this mean?

Well 3,600 sesterces is 900 denarii.

1 denarii couldnt buy you much, it would get you an egg. 2 denarii is how much the barber would charge or a trip to the baths. 1 lemon would cost you 24 denarii. Senators shoes would cost 100 denarii. a pound of fattened goose would be 200 denarii. 30,000 denarii could get you a male slave.* a racehorse would be 100,000 denarii.*

These are based on the price controls to combat inflation under Diocletian.

You can see this is confusing. But basically, to maintain a legionnaire, would cost the above amount of denarii, but it doesn't really mean anything in relation to today.

There was a massive gap in wealth between the average soldier and a member of the elite.

1 pound a emperor's purple silk (150,000 denarii) would cost the same as having 166 legionnaires for the year. ;)

Source: The oxford dictionary of the classical world edited by John Roberts pp839-840.

*these are the numbers given in the book, I assume they are averages but it doesnt say.....