Once you are in out on trek you can subsist on next to nothing, a days accommodation at a trekside lodge and local food cost around £3 to 5 a day. Western snack food such as Mars, Snickers,Pringles, Coca Cola and canned beer are available in most places, the cost rising with altitude! If you can't manage without a Mars bar it will cost you at least 100Rp! There are few opportunities to change money in the Khumbu, to make sure you change enough money in Kathmandu, low denomination notes are the order of the day. Most shops in Namche are happy to take Dollars and usually at a better rate of exchange that at the official money exchanges. Depending on how pessimistic you are, you should also take spare cash in case of emergency. On my Khumbu trek, a colleague was helicoptered out suffering from altitude sickness. Many people suffer minor mishaps such as twisted ankles especially those who are not prepared for snow and ice.
In Kathmandu there are hotels of all standards and prices to match. I stayed at the Royal Singhi whilst I was in Kathmandu, which is a new 3* hotel near the Rani Pokhari. The staff speak English, the breakfasts are reasonable and the rooms are large and airy - no complaints at all. A double room costs £40 a night, a bit steep but I worth it to relax in luxury after the trek. There are plenty of good restaurants and bars in Kathmandu most in Thamel. My favourites were Helenas for its Lemon Meringue pie, the Rum Doodle for its atmosphere and very tasty food especially the Rum Doodle steak and the Pumpernickel bakery for its cinnamon rolls and apple strudel.
The are now many places to phone or email home from Kathmandu. An hour on the Internet costs around 150Rp! I bought a postcard, scanned it at one of the Internet Cafes and e-mailed it to my friends back home.
Tourism is Nepal's main source of income and it is difficult to get away from souvenir stalls even up in the mountains. You will no doubt be tempted to purchase some momentos but how much and how many will be up to you. Everything is available in Kathmandu and Patan so travel light whilst on the trail! I bought a few things whilst I was in the hills. In the Annapurnas I bought a silver Tibetan ring which has been on my right hand ever since that day.
The last expense in Nepal is the Departure Tax which has to be paid at the airport.