I guess it's the law of supply and demand. SDI/HD-SDI chipsets are a relatively niche product (I remember evaluating first generation SDI chipsets in a former life!), so probably significantly more expensive to purchase, in small-ish volumes, than other drivers. (HD-SDI / SDI is pretty sophisticated tech as well - as it is sending high bit rate uncompressed video down much longer single feed coax cable runs than the cheaper HDMI stuff that uses multiple feeds?)
Add into that the volume sales of actual HDMI to HD-SDI/SDI converters - plus the HDCP issues (and possible requirement for scan conversion in the case of SDI) and you aren't going to find dirt-cheap solutions.
For what is a pro requirement (SDI/HD-SDI are pro connection systems) - the sub $500 price range is not unexpected.
We have a number of HDMI to HD-SDI interfaces at work - which do a straight 1080i or 720p pass-through of a non-HDCP source - but they are boards you buy to fit into a standard rack mounting enclosure (and share the PSU and control etc.) Work well for connecting PCs into broadcast infrastructure though.
I've used my Macbook connected to one via a Displayport to HDMI adaptor to play-out off-air HD recordings made in Windows Media Center to allow them to be used for editing and broadcast - under UK 'Fair Dealing - Criticism and Review' rights law. Worked very well. Much easier than trying to import a .wtv file into an Avid or FCP, and hugely higher quality than YouTube...