Call a local laundry service and find out what they would charge, then drop it a bit b/c you will be paying her cash.
Just a couple of thoughts, though...
- Baby spit-up stains, so you will be doing laundry far more than once a week. You can save the non-stained items for her to do, but when my son was an infant I was doing his laundry every-other-day. Unless you have tons of sheets, you will too!
- IF you are going to keep the bulk of it for her to do, make sure you soak the stained items in a water/OxyClean until she picks them up.
This is a big help, but probably not as much work for her as you are imagining. My guess is that an extra $15-$20 per week would cover her gas, time and the water bill at her house.