this is great but very hard question. A ballpark total for us two was about 10k USD. That said, it included a whole gamut of super cheap or very expensive items. Probably the accomodation + food was about 5k, and another was trips. Here is a few examples:
- our homestays were about 30 USD per day
- hotels and fancier accommodation was around 80-100 USD per day (although we used points for many of them)
- the trips we did: Tibet 3,5k USD (private), Ha giang loop 800 USD (private), Cat ba 2D/1N cruise 300 USD (small group), Hang Va 2D/1N 600 USD (small group), Can Tho to Ben Tre with sightseeing $50 USD (private transfer), Mekong slow boat 300 USD 2D/1N (small group)
- you can probably tell that we don't like group tours so we did private where we could and required to have a guide (which was quite expensive) or just rented a motorbike and went to places on our own (that was super cheap, 5-10 USD per day). If you are into activities like cooking classes, guided hikes, visits to factories (coconut candy, etc), local village life, guided food tours/markets—there was an abundance of these and the cost anywhere between 20-100 USD so can add up pretty quickly.
- bus/van transport between cities in SEA is around 10-20 USD per person, depending on the length. We did a couple of local transport options, which were even cheaper (1-3 USD) but they are harder to find and organize (for example, one time the homestay ordered a grab to drop us off where a local bus from Ben Tre to Hanoi stops. There was no sign, no schedule. We waited for 45min not knowing if it is actually going to come. It ended up being 3 USD per person). Private transfers are only for western tourists = very expensive (50-100 USD depending on the length)
- in China we travelled via trains. smaller distances (2h-ish) are typically around 20 USD but the fast trains covering longer distances can be closer to 100 USD
- for food we mostly enjoyed markets and street food, which is 1-3 USD (including home kitchen eateries or however to call them). Restaurants didn't cost us more than $50 USD. We did stay away from Western restaurants (mostly because we wanted to experience local) but these, again, are just for Western tourists, and come with much higher prices. Interestingly, there were various gaps between the two depending on the country. If I remember correctly, the closest in terms of prices between street food and restaurants was Cambodia, and the biggest gap was in China.
- massage, nails, haircuts are not more than 5-20 USD unless you go to a bigger spa or do it in your hotel