That is something that always bugged me.
I recently booked a car in SAN for 10 days. Avis US gave me a quote around 300$ without any insurance. I don't need LDW, but SLI would cost me another 150$.
When I booked the same car at avis.co.uk (just the website - nothing to do with the country of residence), I got it for 270$ (prepaid + Luftansa AWD) including LDW and SLI. Other European AVIS websites (like avis.de) had similar conditions, but a somewhat higer price.
So far avis.co.uk was the best I could find. But does anyone know if there is an even better site somewhere in the world? I.e., one with a good price including SLI (and possibly LDW).
And just a final warning: I used the same trick couple of years ago, but they messed it up and converted the prepayment in UK Pounds 1:1 into US$ and then charged me for the rest. It took a few phone calls to get it refunded.