You should easily be able to charge more than you currently are! The people here who do house visits charge between $20-25 per 30 minute visit ($25-35 for an hour). They also generally add $5 per extra pet, though I think that's silly. The real cost to you is the time to get to their house; adding an extra pet isn't that much extra work for you. Lots of them charge extra for walks too
In order to have someone come to my house it would have been at least $60-75 per day, PLUS the cost of walks. No thank you! I'd hire you in a heartbeat at your prices!