Accommodation prices will vary from Kathmandu and Pokhara to less remote places such as home-stays in the village while trekking.
Typically hostels in Kathmandu are around $4-8 USD for a dorm bed; private rooms in hostels, Airbnbs, or hotels in Kathmandu about $20-40 USD; home-stays within mountain villages only a few dollars, and sometimes included in a trekking package if bought. Most accommodation in Nepal is between $5- $35 USD for backpackers.
A meal out is typically $2-5 USD in Kathmandu and there are much more expensive places in Pokhara. However it is possible to get noodles or simpler meals for $1 at more local places. While trekking, more effort is needed to get food up the mountains so it comes at a cost of around $7 USD.
A flight in between islands can greatly vary depending on when you buy it but could be between $20 – $100 USD. A ferry between islands farther apart is typically around $20 USD, a shorter ride around $5 USD. The cost of renting a motorbike for a day is $6-9 USD.
Trekking permits start at around $20 USD and can go up to hundreds of dollars depending on the region being trekked.
The cost of a porter and a guide can vary.
Budget per day in Nepal depends on if you go trekking and how you go about it (with a guide + porter, etc.).
As a backpacker I would budget at least $30 USD a day. This is not including any trekking, permits, or visa costs.
Although some places in Kathmandu and Pokhara will accept an international credit card, I suggest paying in cash. If you go trekking you absolutely must have cash to pay in the villages.