As a rough guide (for my work) as a "professional pet-sitter" (i.e. I do it as a living - who also does horses.
I charge £10 visit for 1 visit to cats - which is feed, check water and change litter boxes, as well as give medication, along with associated cuddles. 2 visits in one day to the cats, I charge £16 for the 2 visits, so reduced rates for more visits.
Dog walking, I charge £10 per visit - they get a 45 minute walk. If the dog is a regular client, the price drops accordingly depending on how many walks in a week, they get - so if the client books all 5 days M-F, the walk drops to £7/hour. If the client wants a one-off 2 hour walk, the price drops to £16 for 2 hours (so £8 an hour), IYSWIM?
Horses, for holiday livery in their own home, I charge £30 for 1 visit. That includes mucking out, feeding, changing rugs, turning out and exercise/riding. if they need a second visit in the day, I charge £10 on top of the £30 to bring in and put to bed, so £40 for a "full day" (and 2 visits). The reason I do it like that, is because I have several clients who bring in their own horses at night, or chuck them out in the morning, but they just want everything prepared and don't want to do the donkey work.
If the client is outside a 5 mile radius of me, they pay mileage at 30p a mile for extra mileage on top of the 5 miles too.
I then tend to negotiate on top of that - so for example, very good friends of mine who I cat sit for, don't get charged the daily rate, we go out to dinner instead or I do it for a couple of bottles of wine, etc.
Don't undersell your services, seriously. The reason I say that, is that I have dealt with a number of "clients" who, when I started and I was on the cheaper side, they took the piss because they thought it was a "hobby". But remember - if you're going to do it as a good business, you still need to pay professional liabilty insurance, market it, pay for equipment/clothing and pay for mileage.
HTH. I'm in South wales, BTW. :)