how much to charge for a bath?

When I've had clients ask for this I check the pricing in 3 groomers close to their home, then add about $10-15 depending on how large and furry the dog is. By coming to their home and bathing the dog you're offering a service above and beyond what groomers do, so you should be compensated for it.

This is just a suggestion so I would check you local area for pricing :

Small Wonders & Cats $14 Includes most tiny and small breeds such as Beagle, Bichon, Boston Terrier, Cairn Terrier, Chihuahua, Chinese Crested, Dachshund, Fox Terrier, Italian Greyhound, Jack Russell Terrier, King Charles Spaniel, Lhasa Apso, Maltese, Mini Pinscher, Pekinese, Pomeranian, Rat Terrier, Toy Poodle, Pug, Yorkshire Terrier. Also includes most cats and kittens. Guideline: Under 20 pounds

Middle Weights $16 Includes most "average" size dogs such as AmStaff Terrier, small Australian Shepherd, Bassett Hound, Border Collie, small Boxers, Bulldog, Bull Terrier, Cocker Spaniel, English Cocker, Foxhound, Miniature Schnauzer, Shar-Pei. Guideline: 20 to 50 pounds.

Too Large for a Lap Dog $18 Big dogs with relatively smooth coats and/or short hair, including larger Australian Shepherds, Bloodhound, large Boxers, Chesapeake Bay Retriever, Collie, Coonhound, Dalmatian, Doberman, Giant Schnauzer, German Shepherd, Golden Retriever, Greyhound, Irish Setter, Keeshond, Labrador Retriever, Pittbull, Pointer, Rhodesian Ridgeback, Rottweiler, Standard Poodle, Weimaraner. Guideline: 50 to 80 pounds.

Gentle Giants & Thick Coasts $20 Includes the largest breeds and/or ones with thick undercoats such as Afghan, Akita, Bernese Mountain Dog, Chow, Eskimo, Great Dane, Great Pyrenees, Husky, Irish Wolfhound, Malamute, Mastiff, Newfoundland, Sheepdog. Guideline: 80+ pounds and/or Double Coated.