Mark - I estimated cost of Homasote vs. ceiling tiles around $100 - $150. Other considerations were weight of a Homasote sheet and using our geriatric crew to carry them down the stars to the basement. Also, argument was that dust from cutting would require the carrying the Homasote back up the stairs to cut. Granted the sheets are heavy but I cut Homasote for a club members layout with a battery powered saw that created fine dust with a pile of "fluffy" dust (had to do with the speed of the saw and number of teeth).
One big advantage for the ceiling tiles is that they are 2 x 4 feet and very easy to handle. Disadvantage is that the tiles don't have good compressive strength and dent very easily especially when crawling over them (at least with my bulk - guess from consuming of too may nuts). Big question is whether or not the vertical supports will be supported enough over time and use (tried to use as large a footprint as possible). We've experienced compression of the tiles when tightening track screws down (apparently mostly from your truly) and having to shim the track in those areas.
