If you already have some training done and you are part of a club, it will cost you even less. As far as training once or twice per month, you may be able to progress, or you may just be spinning you wheels. When you start flying cross countries, those flights get expensive and they require a high level of frequency to master them. When I was training, I did about one a month and that was a long and expensive process. My experience is the people who fly at least once a week only need about 3 cross countries to before they are ready to solo, and those that go once a month take closer to 10.