All horse mortality policies are based on value, usually 2-4% of the insured value annually (average is around 3.1%). There is usually a minimum premium of $150-300. You have to insure for PURCHASE PRICE if the aquisition is recent, and regardless of what you do you have to justify their value, you can't just pick a number. This policy ONLY pays if the horse dies, assuming that you do a post mortem and do EVERYTHING the vet recommends to prevent their death. If they need a $4000 MRI or a $10,000 surgery, you have to do it or the insurance will not pay out the mortality benefits.
Major medical/surgery is a policy usually added to a mortality policy. This policy pays a certain amount (averaging $7500 but depends greatly on the underwriter as well as which plan you select) for major medical (colic, lameness, etc) expenses as well as surgical expenses (colic, broken leg, etc). This may or may not have a deductable (usually $250-500, many are $0) and will exclude that paticular body part in the future (ie if you have colic surgery once, they won't cover it next time). This policy is a fixed fee with fixed coverage regardless of the horse's value.
Surgical riders is generally something you want to avoid... they are cheap, low coverage $$ levels ($5000 or less usually), and have hidden pitfalls. For example, Great American's policy is $75 for $5000, but that money can ONLY be used for SURGICAL expenses... note that anestesia, post surgical care, etc is NOT considered part of the surgical charge... ONLY the surgeon's time is paid for. In a $7000 colic surgery, about $750 is the surgeon. Again, this rider is added to a mortality policy, you can't have it without the mortality.
Loss of use, fertility, etc are optional clauses you can add, which significantly add to the policy's price (usually doubling it). Most of us have no use for these policies, but they are offered.
I have used three companies in the past, all did minis as well as full sized breeds. The percentage fees vary based on breed and use (ie minis are higher than quarter horses, jumpers are higher than pleasure horses, eventers are higher than all, racers are usually not covered). Markel offers a liability rider which is nice, but usually not needed due to other insurance coverage (my renter's insurance's liability insurance covers the horses as well). Great American has good coverage but difficult customer support. Now I use Travelers and so far am much happier with tem than anyone else.
Also see this post from last week for more information.
http://www.miniaturehorsetalk.com/index.php?showtopic=100295
