This one is used quite a bit by others here and I've found it to be helpful:
RSMeans Landscape Cost Data
It breaks down tasks like planting a shrub down to the gallon size of the plant, whether it's B&B or potted, or for trees, the size of the hole dug, etc. And then gives the number of minutes it should take. You can take the data and plug it straight into a spreadsheet to help with bids.
