For medical funds it is important to understand how much some things can cost, which can vary wildly vet to vet. In my area just a consultation with my vet it is $60, and that is before any tests or medications. Some diagnostics can be expensive, sometimes just getting an x-ray, which can be necessary for diagnosing some common health complications such as bladder stones or teeth issues, can sometimes cost somewhere from $100 to $250 as just a ballpark figure.
It really depends on how cheap/expensive your exotic vet is that determines how much is a good amount to have set aside. Personally I like having $500 per guinea pig set aside for medical funds, or if you have just one pair I would probably have about $700 per guinea pig set aside. That way if one were to get an illness that required diagnostics then surgery I would have enough on hand to cover it along with any follow up care. If multiple guinea pigs got sick, for example with something contagious like Cervical Lymphadenitis, I would hopefully have enough to cover the medical care for all of them. I find the idea of having more then I strictly need set aside better than the idea of not having enough and not being able to get them the care they need (or better than having to rely on backup methods such as a credit card or loaning money from friends/family).
You don't need to have it all at once, I started with a vet fund of about $150 total (enough to treat 1 guinea pig for a URI at my vet), with my credit card as backup for if something were to occur while my vet fund was not large enough. Then I just started by putting a little bit aside each pay to add into the vet fund until I felt I had enough in the vet fund to cover most situations, now I only start putting money into it after I have used some of the vet fund. I also only touch that fund for vet care, unless otherwise absolutely necessary.
