The best months to trek to Everest base camp are pre-monsoon (February, March, April and May) and post monsoon (late September, October, November and December) Prices range per person from USD$1,400 up to $1,700 for an average 12 - 15 day trek. Prices should include: permits, round trip flights from Kathmandu to Lukla, accommodation, meals For physical preparation , start with a sensible distance that you're fairly comfortable with and work up from there. Before this trek try to do at least three long distance day hikes – 5-6 hours minimum. If you can do this without too much fuss then you'll be in great stead for your Everest trek.
You can look up to certain organizations such as Indiahikes, Trekthehimalayas, Bikat Advetures, Renok Adventure & EliteHimalayanadventures. The cost depends on the agencies. On an average it would cost 40K.