A new 100 A service panel runs between 1500-2000 (installed by electrician cost) usually. You can save some money by only running a 50 A panel, or by replacing breakers in the existing panel with tandem breakers. Depending on how many extra circuits you need, the tandem breakers may put you in violation of the NEC, which is a big no-no. If you can list the quantity rating and use of your existing breakers, the rating of the panelboard, and how many new circuits you will need, I can verify whether or not the tandem breakers will meet code.