is it really $70 per bag ?
I found this helps quite a bit, helps shade the lawn and reduce crab grass germination, also forces the crab grass that does germinate to grow vertically upward and not horizontally outward.
The other thing worth mentioning is the crab grass preventer is a barrier that can be "broken" by disturbing the soil by raking, etc., so once the barrier is down don't disturb the soil