They are definitely pricey. Part of the high cost is the fact that they can be SBR'd and have a registered sear put in to basically make it a full-auto MP5.
All original HK-94s are becoming more scarce. Keeping one in original condition with the long barrel I think will make it more valuable in the future.
Here's a photo of my MP5:
It started life as a 94, but was SBR'd, had a paddle mag release installed, and a sear put in. It was remarked to MP5, so an uneducated person would not even know that it was originally an HK-94.