Hi! By floor mats I'm guessing you mean rubber mats?
I do have shavings on top, but you don't need as big a bed because the floor is softer on the ponies's feet. Simply from experience I've found banks are still needed :)
For one horse I use about 2 bags a week, but I use quite large bags and the smaller, and cheaper ones I generally use 4 a week. I use the bigger bags, even though more expensive because they are better quality, last longer and are 99.9% dust-free. In a month anything ranging from 8-10 bags a month, because we get ours delivered weekly I'm not entirely sure.
We get ours delivered so the price is around £8 a bag (sorry I don't know what that is in dollars) and £10 when they are delivered. We have 2 horses, both on mats so we get around 4 bags a week and occasionally 5, depending on how dirty the horse's were and also if we need any for the trailer.
I don't know about America but either DuoBed (a mix of about 40% rape straw and about 80% woodchippings) or StableChoice (which is the good-quality, dust-free shavings our horses are now on) and both are very effective. StableChoice is finer, softer and better for winter when the horses don't get grazing time but often we use DuoBed in summer because it's cheaper and the we don't have to worry as much about respiratory.
I clean Dusty's stable out daily and take about 1-2 wheelbarrows out, so generally I put down half a bag every other day. Shannon is much dirtier but has thicker matting and therefore requires a thinner bed (this is to save money).
Stable mats, are a highly expensive option but they generally last a long time and means a cheaper bedding.
It is still necessary however to disinfect the stable every now and again and this job is made significantly harder with mats as they are extremely heavy!
This is all I know about them, mostly from personal experience. Good luck - I think they're fantastic!