I've never had a problem getting insurance at the border of ukraine or anywhere else, as long as its not some small outpost type border that gets 4-5 people crossing per day. (this happened to me once crossing into serbia from MK) If you go down the croatian coast, you only go thru about 20km of bosnia, dont worry about Ins. there, as they have never asked me for it. Last year, I think ALB ins cost me about $30? for a month.
serbia is expensive, about 100$ for a month, macedonia, 50 euros for 2 weeks. Ukraine was about 25$ a month.
Im in Bulgaria for the summer, and can get normal green card just for EU for about 40 a month. or I can get green card that covers everywhere for about 125 a month, on my US registered bike.
Hope that helps