Vicky made an important point - keep money to one side, in a "vet fund", solely for any vets bills you may have to pay out for your pig. I had a rough two years with illnesses with my then-seven piggies and paid out around £900 during that time.
Even just putting away £10 a month will help.
Also, be sure to find a good, experienced guinea pig vet (or rodentologist) before you need them. You do not want to suddenly have a very sick piggie and then discover that your vets are useless and you don't agree/like/get on with them. Find someone as local as possible, who you trust and like, who listens to you (very important) and who actually shows some caring towards your guinea pig. A vet who respects an animal as well as who treats their illnesses is usually a good one.