Are you planning to visit only Jomsom or up to Muktinath? If you are planning to visit up to Muktinath this (http://www.yentra.com/question/94#reply2) might help you about the days to reach there. You can plan it accordingly. Or you can take extra 1-2 days more or less according to your time availability and budget. Rs. 1,500-2,000/ day for one people will be enough if you are going to eat normal breakfast/lunch/dinner. But it is always good to keep some extra money as a backup just in case you need in emergency.
Sept-Nov is considered as a good season to go there, but you can also go there during winter in Feb-May. Try to avoid going there in rainy season or during the heavy snowfall.
I hope this will be helpful to for your journey. Don't forget to share your experience with us.