In my experience, flying from Washington, DC, figure on a round trip, Open Jaw" airfare of about USD 1,300 (+/- 100), and another USD 100 - 125 to get from CDG to St. Jean Pied de Port by coach and rail.
Once at St. Jean Pied de Port, plan on an AVERAGE of Euro 30-35 per day if you are staying at albergues (combined public and private).
If you stay at a mixture of private albergues and hostals /B&Bs, figure on an AVERAGE of Euro 50 per day.
If you fly Open Jaw home from Madrid, as I usually recommend, figure on another USD 100 to take the ALSA coach to the airport from Santiago, and another USD 150 for a hotel near the airport overnight so you can get your return flight the next morning.
To recap:
1,300 (USD) r/t airfare JFK-CDG // MAD-JFK
950 (Euro)
125 (Euro) ground transportation to CDG St Jean Pied de Port
1,750 (Euro) HIGH average daily cost assuming 35 days en-route (St. Jean to Santiago)
250 (Euro) ground transportation SdC to MAD and one night @ airport hotel.
3,075 (Euro)
Using today's conversion rate, the r/t airfare works out to Euro 950
Also, using that same conversion rate, the 3,075 total works out to about USD 4,200.
I purposely built in the HIGH average to ensure you had enough funds while in Spain. You can likely save at least 500 Euros off these estimates. However, one thing you learn early on is that EVERYONE walks their own Camino. So, these estimates are just that, a starting place. I intentionally erred on the high side.
I hope this helps.