Agree with Kendalle. There is also not a fix to this that doesn't involve vet care. Pets require vet care. End of story. "Costing a lot" is not an acceptable reason to deny health care to your pet.
How would YOU like to have something wrong with you that you couldn't describe but have your caretaker decide that you weren't worth the money?
A competent vet should at least be able to rule out several major causes of weight loss and come up with a treatment plan. Even though the front teeth look fine, their molars could be overgrown. It's nearly impossible to visualize these without either an otoscope or buccal pad separators, so there's really no way for an owner to check. They're also much more common to overgrown that the incisors, and the incisors VERY rarely overgrow without molar problems in conjunction.
There are a variety of other problems which can cause weight loss, unfortunately it's a common symptoms of literally almost every ailment a guinea pig can have since most pigs will either stop or slow down eating when they're in pain or don't feel well.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry