As you can see their are many alternatives for mat board sheets. Generally speaking you get the lowest cost for mat board by purchasing full sheets by the box. Most companies will give you good discounts for lower quantities. For example our company gives 17% discount for qty 5 and 24% discount for box qty (25 sheets) for Bainbridge AlphaRag and Alphamat matboard.
The downside buying online from companies like ours will be the shipping cost and possible shipping damage. Some companies will save a little on the shipping by shipping the mat board in the original manufacture box which means that you are virtually guaranteed to have some corners damaged. Companies like our pack that box in a larger box to help protect the corners but the larger box does increase the shipping cost. Different companies may be closer to your location so you will want to check the total cost with shipping before making a final decision. If you are not cutting 16x20 sheets out of the mat board you may not be concerned about corner damage. Also, if you are not in a rush, you may not mind waiting for replacement mat board.
If you need a lot of mats at the same size you might consider buying pre-cut. When you buy pre-cut at qty 10-25 the discounts are high. Shipping cost is lower and you may find that although it takes a lot of time to cut yourself, that you do not really save enough money to make it worth while. Computerized mat cutters can provide quality that it is difficult to match with by hand especially on 8-Ply rag board.