Just the SFP is 140$ not to mention that the converter box is about 100$ to 200$
Right now we're using Level One FVS-3800 converters with Digitus SFPs. The converter (which is remotely manageable and supports bandwidth limiting, VLAN, Q-in-Q) costs about 76 Euros and the SFP about 22 (excl. tax). There's also a Gbit version with the GVS-3800 with about the same price tag.
The FVS to us proved to be rock stable. We started deployment in early 2012 and never had a problem with them, other than I'd rather use something rack-mountable with more features per buck - such as an RB2011 if only it'd support 100FX SFPs.