Airfare: the easiest way to get to the Camino is to land in Madrid. There are trains and busses direct to where ever you are starting from.
Train/bus to the start: from Madrid, you'll need to take a bus or train (they both take the same amount of time) and cost roughly $20 based on when you book it.
After the camino: Santiago has a great airport so you can fly back to Madrid, on to Paris, London, Rome, wherever your heart desires. There are also trains and buses.
Accommodation: here's where costs can vary. You can stay in albergues which are hostels with bunkbeds and shared bathrooms ( roughly $7/night); there are small hotels and you can book your own room (roughly on average $30-50 for two people) and in some places you can get into some really nice, expensive places (the sky's the limit). See the Accommodation page for more details.
Food: Most if not all of our meals and refreshments will be at bars - that's the mainstay on the Camino (and in Spain in general). You might get breakfast at your accommodation or at the first bar you pass in the morning. Then there's second breakfast. Everything is very inexpensive on the camino - between breakfast, lunch, dinner and snacks, I can't see you spending more than $20-$30 a day at most - depends on how much you drink! Many dinners are "Pilgrim Menu" which cost $10 and are three courses with a bottle of wine. Usually in the evening we shop for snacks to have in our backpacks for the next day's walking.
Luggage transfer: I've devoted another page to this, but if you send a bag ahead every day, expect that to be roughly $6.00 a day.
Beyond that, there really is nothing more to spend your money on. You certainly don't want to buy anything because you'd have to carry it!
June, 2019 dinner in Santiago de Compostela