IMHO it may be that you're getting conflicting answers because of the way you're phrasing your question.....this is an observation NOT a criticism, so please don't flame me!!
Perhaps I am not phrasing the question correctly...if so I apologize and have slapped myself in the hand with a ruler!
1. If you book via a
travel agent on a fly-drive then without question you get charged over the odds, it's called profiteering....but you can't blame the travel companies for making easy cash where thay can!
Yes,this unfortunately seems to be the most common method although I wonder if most folk are aware of the choices they have (ie

IY rather than package).
BTW...I do blame the travel companies for profiteering and I think blacklisting them on these boards would be called for.
2. I can't imagine too many UK car insurance companies would be interested in going through the hassle of arranging insurance in the US, not least because of the worries of the nonsensical figures for damages claimed in the US courts. In fact would it be any cheaper by the time they are satisfied they comply with US legislation..?? I'm sure the UK car insurance companies have probably researched the market and made their decision..?!
I'm guessing that you are probably correct on this one and I am still waiting for my relative to reply to this very question.
3. The alternative is to arrange your own car hire and insurance . There's a huge choice of companies who will rent cars and full insurance to UK visitors......................I've been getting quotes for a 4-door Intermediate for £330 for TWO weeks, that's with all insurances, taxes etc included...... so £300 plus for one week would seem high. I think Vernon offered very good advice ( I followed it re : car rental ! ) about checking costs out. Can I suggest going to Google and entering " Florida car rental " ...?
I think you did OK with your quoted car hire rate.
Unfortunately I don't think most of your fellow citizens are researching to the extent that you obviously did (at least not as far as the message threads I've read on these boards is concerned).
The Google search will provide many choices but I think some folk tend to jump as the first "pretty" website they come across rather than taking the time to delve into it further.
If, as you say. one of your family members claims it IS possible to obtain out of country insurance why not ask him/her to get the facts and post the info on the boards, that would be a great help to many members here?........and save you getting conflicting answers?
As I mentioned earlier,I am still waiting to hear from him and I will post any positive responses.
As for me getting "conflicting answers"...
Thats what these boards are for,getting answers to questions whether they be conflicting or not!
I'll finish my marathon message by enclosing an email message I got from a company called
http://www.travelguard.com/.
Anyone in the world can purchase our insurance. Our collision damage wavers will be one set price either $7 a day for $25,000 of coverage or $9 a day for $35,000 of coverage. Thank you for your inquiry if you should need any other assistance the # to reach us at is 1-800-826-1300. Thank you, have a great day. Amber