Wow - These are cheap. I paid $23.00 for 30 minute walk in Bethesda. They had a key to my house.
Since we have a multi-generaltional house and there is almost always a family member home to walk the dog, I only needed a dog-walker for about two months when our parents were out of town. We walked her in the morning, and I used school aged kids for after school and they came very cheap (my neighbors' kids!). But it was that noon walk that I needed a professional because all the kids were at school.
We used A Walk in the Park. They were good and trustworthy - but obviously expensive!