As others have said here, it depends on what you want to do and how much you want to spend. For a good piecing workhorse, nothing beats a Singer 221 Featherweight (the original models manufactured between the 1930s and the 1960s). It has a straight stitch, goes forward and back, and is easy to maintain. The cost varies depending on who's selling and how much you want to pay.
I started sewing at age 12 on my mom's Kenmore, bought my first machine (a Singer 237 FashionMate) in 1971 and it still works. I upgraded to a Bernina 440QE (without the embroidery module) about 3 years ago, because I wanted some decorative stitches. It cost me $3,000. The two features I like the best are the knee lift and the needle up/down.
My suggestion is to visit dealers and sit down and sew. Ask about repairs. And see if they have classes to teach you the bells and whistles of your machine.
Good luck and happy hunting!