Oh No - Not BIEBER!!!! :-)
Just a brief note... The D-cell powered ELTs are strictly 121.5MHz units certified under TSO C91 or C91a, not the 406MHz units certified under TSO C126 and its later variants.
Thanks, Scott for citing chapter and verse. There are exceptions to the "no TSO required" generality for Experimental aircraft, and ELT's are definitely one of them. Non-TSO'd ELTs simply don't meet the requirements of the FAR as quoted by Scott. Using non-approved batteries invalidates the TSO, therefore an ELT with unapproved batteries installed is in violation of the FARs. This same story goes for 121.5MHz units as well as 406MHz units - we can only install batteries approved by the ELT manufacturer.
Coming back to the OP's point, yes, the approved batteries are expensive, but they do last for 5 years or more, so the average annual cost isn't terribly much more than the battery packs we are accustomed to buying for our older 121.5MHz ELTs. The exceptions as pointed out previously are the D-cell-powered units, cuz there ain't gonna be a cheaper ELT battery than good ole' copper tops!