Whooa there S-Mendus, if you are serious about wanting to save the dinero my suggestion would be to hold off on buying insurance on-line at the rate you mentioned. Better that you wait until the day you are actually at the border and shop around in person at some of the insurance sellers on the USA side . You should be able to do a lot better.
This past winter I made two trips into Mexico and got liability at lower cost than your example. In January I paid $75.16US for 21 days of cover for an automobile at a seller in San Ysidro California ( at the Tijuana entry, the actual name is not in my expense notes , but I can find it later) and I paid $113 US for cover for my bike from 15 March to 17 April at the Sanborns agent Bravo Insurance in Laredo Texas. In Laredo I did ask at another seller too but they were a lot more.
I know, rates may have increased in the last two months, but for the longer your stay the rate does go down per day.
I suspect that rates are higher over the net because they charge for the "convenience". . Better to not over plan a trip, just buy the insurance when you actually need it and you also have the benefit of having a more flexible schedule. There is no refund on such policies so if you buy it you are stuck to the start date or know that you are wasting policy time if you start late.Much to be saved if you take the time to shop in person during business hours. All the big and small border towns in the US will have one or another insurance seller. If you can head for the big cities where there is competition