Strength training

I have been working toward progress for almost 3 weeks now. I have built up my endurance on the elliptical from 5 to 45 minutes.

I also do turbo jam on non elliptical days and Pilates.

I don't have a lot of extra money to join a gym and noticed that weights increase in price the heavier the weights get. So I have been doing a lot of stuff with a 4lb weighted ball I have, and push ups and sit ups, along with reverse crunches and other AB exercises


Are there any good non weighted strength training, or strength training that involves cheaper weights? I have 2 kids in school and sports and they take a good chunk of any extra cash I have.

I have attempted lunges and squats but those kill my knees.