Sixt is in the Rental Car Centre with all the other major car rental companies so is off site but so are all the others.
The RCC is brilliant, helpful signage, information desks and useful people are banned by state law in FL so you sort of just have to wander around until you see a funny shaped shuttle bus. The pavements (they call them sidewalks) are specially designed at MIA so no matter where you leave your trolley, it will roll into the road and hit a car.
$15 sounds like the going rate. I got a whole Dodge Charger for $30 per day from Thrifty. I just booked a compact for $12 next week through Alamo.
Be careful when they try to sell you extras at these places, the Thrifty guy told me that I would need to buy a SunPass too, I told him I was going up the I-95 and didn't need one, he said oh you sure do, there is a toll gate and they no longer accept cash. I didn't believe him and was right not to, they do accept cash on the I-95 and it is about $1.75 which turned out to be exactly the amount of money I had in my sock