Then yes, that's an excellent price. Here's the math:
4' (48" ) x 25' (300" ) = 14400 sq. inches
Using a 7 x 12 board as your base board size = 84 sq inches
14400 / 84 = 171 total boards in a sheet (very rough estimate; you'll likely have somewhat less depending on how you cut them -- as alaskasunshine pointed out, if you use a tablesaw you will lose some width due to the blade. If you choose to go wider than 7" then that will also affect the number of total boards as well)
$25 / 171 = $0.15 per board.
That's the cheapest yet -- and shows how if you can buy locally, and have the resources to cut sheets of that size, then you can definitely do this very inexpensively!
Sorry if all this talk of price per board is boring to anyone :) I'm a programmer, so I just have to break everything down as logically and mathematically as possible ;)