i guess first up, you need the u13 calipers to work with the u13 rotors. thought i'd just point that out lol.
as per what douce is saying, if you buy something cheap, chances are it is. i have recently bought rda/ebc brakes for cheap on ebay though, but it was an end of stock special in which they were marked down by the vendor.
all those really cheap (around/under $200) slotted/drilled rotors from melbourne, i would be very wary of them. its awfully tempting, but as you say, probably not worth the risk. the main ebay shop i'm talking about is
this one but there are others like it.
if you buy the brands though, there is usually nothing wrong with them. you will find that a lot of the time it is a small automotive shop selling at their trade price.