My high watermark on the rewards scheme is around £400, but monthluy varies from £100-£300, although I have backed off a lot in the last few weeks due to work commitments.
It's a big number and cost, but I ensure that my selling price and sell-through rate easily covers it.
My monthly ebay fee, including the basic store, typically runs between 12% and 16% of my sales. I usually sell between $1200 and $1500 gross...my fees run $120 to $160.