I before got interferon injection for my hepatitis b 3 to 4 years ago but its too expensive is there any other alternative that is cheaper?

I doubt. All the anti-viral meds are, unfortunately, very expensive as all of them are brand names. I am positive that there are different manufacturer and state sponsored programs that may help you pay for the meds. When i was doing my training for hep. C treatment, drug reps were trying to assure us that they will help any uninsured patient get his meds. Try them. It doesn't hurt to ask!
Yes. Interferon for hepatitis b is given for a finite duration. Either it works to cause the virus to go into remission or it doesn't. If continued treatment is required, there are many antiviral agents that are effective. While these are much cheaper than interferon, there still very expensive and treatment may be lifelong. There are several manufacture assistance program available.