Basic firmware updates don't cost anything. If you want to upgrade the scanner for ProVoice/NXDN/DMR that does have a cost. If all you monitor is ARMER, you won't require any of those upgrades.
You weren't wrong to get a different scanner. The 396 has certain limitations on memory and trying to program the entire ARMER system will exceed those limits.
Sentinel cannot be accused of being "robust software". I don't think Uniden minds that either as they are a hardware manufacturer and "down and dirty" software is needed to get the product going. However, ProScan or Butel are software developers and their whole life is creating "robust software".