Jack, I have an agreed value collector policy on my '66 GTO. You're right in that if the value isn't that much, photos and whatever documentation you have is sufficient, and no appraisal is required. The higher the agreed value, the more likely it is that they'll require appraisal. They also automatically increase the "agreed value" every year. Policy premiums are very very reasonable - however there are restrictions on use, which include mileage, prohibitions on using the vehicle as a "normal" vehicle (such as, not allowed to use it to go to/from work, run errands, go shopping, etc). You must have a fully registered, insured "regular" vehicle for every licensed driver in your house in addition to the "collector" vehicle you want to insure. There are also often age restrictions where any driver of the collector must be at least 23-25 yrs old. As you mention, typically these policies all require that the vehicle be formally considered a "classic" or "antique". The biggest "Agreed value" companies out there, or at least the most common, are Grundy, Hagerty, and Heacock. I've used all of them at one time or another for various reasons.
If you do happen to have at least one "collector car", then Grundy has an MVP program where you can get agreed value for all your vehicles, including current late model stuff and motorcycles. The policy covers everything, but is based on you having at least that one collector car first. Hagerty does have "motorsports" insurance, and our bikes "might" qualify. I don't personally know anybody using it.
Jack, for the mileage restrictions, most of the time since virtually all of the collector vehicles force you to prove that you have a fully registered and insured daily driver for every licensed driver in the house, they don't see much risk and the mileage restriction is a hold-over from years past - when they did do things like force you to get your odometer read by licensed shops, etc.
As mentioned, prices are typically very low for the collector stuff. A $30k vehicle might have a $300 or so annual premium with very very good coverage. A $110k vehicle premium seems to be around $700 a year. Obviously, your zip code will have an impact, and probably your driving record as well.