From what I understand, any type of human shampoo (including baby shampoo) will dry out a pigs skin, and is really not a good idea to use on them because it is so harsh and could irritate them (i.e give them dandruff and make them itchy, etc.). I personally use ferret shampoo on my boys as it is formulated specially for small animal skin and for deodorizing, but I have also heard that kitten shampoo is also ok. I can absolutely understand you wanting to save money--our pets can rack up quite the bill if we aren't careful, right? Personally, I buy my shampoo from WalMart (I know, I know...but I'm a poor student, give me a break guys =) ) and it only costs $5 CAD, which would be $3-4 American; around the same price as human shampoo. If you shop around a bit, then in time you'll find good deals for practically everything you need.
I know my pigs HATE water, as do most pigs. You can always try to give them a bath and see what they think, but I would avoid the bath bomb. As it contains citric acid and baking soda, the combination could also dry out their skin and lead to irritation. If you find your pigs like water, great! If not, then try to limit their baths as it only causes unnecessary stress on them.
Good luck!