We're with Direct Line and bought the advanced policy. For each condition we have £6000 of cover = $12,000 USD. I called yesterday to see where we're at and we've already claimed just under £3000 and have a further claim for about £500 on the way to them. With meds and tests it soon mounts up - her meds are about £250 each quarter so about £1000 a year, plus tests twice a year at about £400 so about £1800 a year this past couple of years. When Lucy went on vetmedin we were told there were cheaper drugs available but that it was the best one so if we were insured, we should go for it.
If your dog is young, and changing would mean no loss of cover (ie pre existing conditions etc) then I would try and get as comprehensive cover as you can afford - it makes life much less stressful later on. With the genetic things that cavs can have - like MVD, SM etc, it really can add up! Obviously I don't know how much drugs and tests cost in the US - these are the UK prices.
Our premium started at about £14 a month. As the claims have added up, each year it has increased. Currently about £25 a month. So we pay out about £300 a year in insurance ... but get back 5-6 times that in claims.
Ideally the best kind of insurance IMO is the one where cover renews each year - whereas we have the lifetime limit. But it depends on your budget and needs. It's worth looking around to see what you can get for your money - you'd be surprised how much policies differ!
Here's a link to our policy type - pdf file - http://www.directline.com/pet/pet_advanced_policy.pdf
Hope this helps
Proud Mum to cavalier Lucy