According to answers.com, there are 64 packets worth of yeast in one pound. The packets are sold in 3 packs for 99 cents to $1.25 at my HEB. So, if you were to buy one pound of yeast in these little packs, you would need to buy 22 of these little 3 packs for $21.78 (and will have a gazillion little packs to throw away).
The one pound brick? Costs $2.74 – a tenth of the price. I you are only going to bake one or two loaves of bread, get the packs. If you are going to bake at least 4 loaves, or 4 pizzas, or 4 batches of dinner rolls; then get the pound. It will keep for just about ever in the freezer.