Dogs with unknown backgrounds can rack up big vet bills. My Lady is a rescue and was healthy when I adopted her at age four. When she turned six her "genetic time bombs" started going off and I now spend about $5,000 a year on medications and routine tests to monitor her various health problems. That does not include unexpected vet visits which usually add another $1,000 to that figure.
Good for you for thinking long term, especially when you already have one. If you don't think you can afford another one, can you contact a rescue group? Or perhaps your vet could help you find her a wonderful home?