Exact weights depend on sizes and there are variations with any one item manufactured. All I can say is that I have weighed several of my various base layer items. In general, the 100-wt fleece is about the lightest, however it is bulkier. When I am really squeezed by space I will use a poly-pro layer because it packs smaller. The wool is the heaviest by a few ounces but I prefer it because it breaths so much better and I think it has a wider comfort range at various temperatures. I have a very light weight merino wool (underware from Sierra Trading Post) that is only 2 oz. heaver than my fleece, and heavier wool (Ibex) that is about 4 oz heaver. The heavier wool is obviously warmer than 100-wt fleece. I am lucky in that I do not get itchy from wool. 100-wt fleece is also much cheaper than wool, if cost is a factor. I tend not to like polypro (or equivilent - such as the Pagagonia stuff) because it smells and does not breath well. Bottom line - I have all three - wool, fleece and poly-pro, and each fits specific needs.

Also, if all you need is a really light layer, you may want to try silk. It is very compact and light, but not as warm.