I think there is some misunderstanding somewhere. The lowest paying assignment I've ever heard of paid much more than $250 take home.
In the not so distant past, Supplemental paid on the high side of agencies (unless you opted more recently for their very expensive health insurance). If your recruiter cannot explain it to your satisfaction, you need to talk to someone else. It is just barely possible that he was referring to your take-home taxable pay at a low base pay. With Supplemental, there is sure to be per diems, housing stipend, travel pay, and license reimbursement, all tax free adding to your total. Possibly even car rental depending on what you need. If they are supplying housing (I think they try hard not to), get what they would pay you for a housing stipend instead so you can better understand what they are paying you.
If you are having trouble understanding the numbers he is throwing at you, collect them all and ask about the tax status of each item. Next use the free Travelers Calculator on PanTravelers to figure out what they really mean to you in terms of take home, or even better, total pay. Then you can compare offers from other agencies to find out if they are in the right ballpark or not.