Short answer: $3 — Bangkok ($1) + walking ($0) + street food ($1) + hostel dorm ($1). Plus a free temple visit.
Theoretical minimum: $1 city (Bangkok/Hanoi/etc) + $0 walking + $1 street food + $1 hostel dorm = $3 total, leaving $7 unspent. Realistic: $1 city + $1 transport + $1 food + $1 hostel = $4. With one $1 activity (night market) = $5. Plenty of room left. See cheapest city Q&A.