"Doing the simple math, I could do $25,000 in coverage (and by the way, the rider also includes lost/damaged jewlery, electronics, etc) for about $12 a month"
Not doubting your word, but I would suggest double checking the inclusion of other items on a rider. Riders, or more correctly, inland marine floater policies generally cover specific items. For example, they will ask for a list of firearms and the value of each. Same goes for jewelry. Usually, serial numbers of firearms are not required to be submitted, but as much detail as possible should be provided. Beware of blanket coverage, as this is often limited to a max of $1000 per item. Beware of coverage where a safe discount is given as the item may not be covered when it isn't in the safe.
I have been an insurance agent for the past 22 years, and have written many of these policies. The areas of typical homeowners policies where items have limitations of coverage (in addition to firearms) are usually jewelry, watches, furs, silverware, tools, cash and bearer bonds. This limitation typically only applies to theft losses. I say typically and usually, because there is no set regulation of insurance on a federal level.
Each state has it's own insurance regulations and policy forms.
$800 plus may be high, depending on your location. I would definetly check with NRA, and also google firearms insurance. There is a policy that I have seen offered at gun shows that seemed to be really reasonable.
Good luck and hope this helps!