i only really cover commercial cleaning but i only carry out CRB checks with my staff that work in schools, these are the enhanced ones and cost me about £45 each...and can take months !!! you can do a disclosure of scotland which you will get back in about 1 week and costs £23 aprox, but only covers issues within the last few years, spent issues will not show....like you say though, you may take on a bad egg but they just have not been caught.
I think if it was me i would get references from previous employers and ensure my insurance covered me incase of theft etc.....hope this helps...xx