I certainly wouldn't pay up front for an SFI visit (Be is my ISP too).
Fair enough if, like kitz said, you wished to expedite the SFI visit, but not for an SFI visit after completing all diagnostics the ISP had asked me to complete. It simply isn't fair or right if an ISP passes this charge onto consumers IF the consumer has complied with ALL requests from the ISP.
A thought that's just struck me - do ISPs receive guidance from BTo, or preferably concrete diagnosis steps, that the ISP should be obliged to follow, before requesting a SFI visit?