Most companies have a minimum on their policy, and I find that to be around $200.00 That is for mortality insurance. Add major medical and that adds some more premium - depending on how much medical you buy. Some are an additional $250 for a standard amount of $7500 or $10,000 (will reimburse your medical expenses to that amount for covered medical procedures). Unlikely you will find medical coverage alone. You MIGHT find surgical coverage, but most of the time both medical and surgical come with mortality.
You can also just get limited /named perils, where the ins. co. pays for death by accident, fire, lightning, transportation, and some others, but does not pay for death by colic or illness. You can normally insure for this for $75-$200 per animal. Depends on the company on how much the cost is, just like for your car.
All depends on how much your animal is worth and how much you want to spend to insure them. A minimum of $500 for a year of mortality w/ major medical isn't much to spend if you purchased for $75,000.00. But you will normally have to prove to the insurance company WHY the horse is worth $75,000 if that is not the purchase price (show or breeding records)
You can't buy a $350 pony and want $10,000 in insurance. That is like me buying a $350 car and deciding I want $10,000 for insurance. But if I buy that $350 car and put $10,000 in it in customization (training for pony- they usually allow 60% or so in training fees for a few years) so it is now worth $25,000 at Barretts auction, then the insurance company will likely not have a problem with my requested amount.
Is it worth it? Only you can decide that.
One other thing - READ YOUR POLICY carefully. Coverages differ by company and also by WHAT you purchased. A limited policy MAY NOT pay if your pony dies from colic. A mortality policy MAY NOT pay if you didn't use the vet's advice to try and save the pony.