The premium is normally 10% of your horse's value. For example, if he's worth $5,000, your yearly premium would be $500. I have the whole smash, major medical, accident and mortality. I purchased insurance mostly to cover the cost of colic surgery should it ever happen. I don't have that kind of money lying around. I've already used it twice for major medical claims. If I didn't have insurance, I'd be out about $4000 to date.
I would suggest calling a few companies for quotes. Confirm what it covers. Be aware that once you file a claim on something, they will never cover anything in that category ever again. I filed for EPM last year, so now I can never get anything neurological covered in the future.
If you're financially challenged, I highly recommend having insurance. There's just too much that can go wrong.