You can also make your own mixes if you are careful to balance it for your pets nutritional needs. Rodents can and do get sick or injured. If they do, it is your responsibility to properly care for them and this usually means seeing a vet. If your animal broke a bone, was seriously injured, needs tumour removal etc and needs to be operated on or Xray'd expect to spend much more money. Unexpected medical emergencies, and colds are all unfortunate but common aspects of pet ownership.
Mites sometimes come in via wood shavings and require treatment.
Fleas are not very common fortunatly, because unlike cats or dogs your rats do not go outside. However, if there were fleas at the store or breeder you bought the rat from you can bring in this parasite. Damages from letting rodents run around the house unsupervised can become costly- imagine dealign with a power chord for your computer beign chewed through. Avoid such problems by always surprising your pets when out of the cage as well as takign precautions just in case your pet gets loose when you arent around- soem peopel hide electrical wires in PVC tubing to protect it. Depending on where you live, you may not be permitted to have pets.