The challenge with budgeting for China is that the country is massive and is effectively 2 countries: developed cities along the coast and poor cities inland. The cost will vary a lot depending on how much of each you want to see and how many places you want to visit
That being said, my rule of thumb for traveling in China is to budget $80-120 USD per person per day, excluding your return flights. This budget should give you a comfortable, though not luxurious, trip to China
Here's ballpark range of costs for different expense categories:
Food - international fast food: ¥50-¥100 ($7-$15) per meal
Food - Chinese fast food: ¥10-¥30 ($1.5-$4.5) per meal
Food - average restaurant/food court: ¥80-¥160 per meal on the coast, ¥50-¥100 in the interior
Taxi: most of your taxi trips within a city limit should cost you between ¥25-¥50 ($3.5-$7) per trip. Taxis to airports will generally cost you between ¥100-¥200 ($15-$30) per trip
Metro: all major cities and many second tier cities have metro and they're usually extremely cheap. In most parts of China you can take the metro anywhere in the city for ¥2-¥5 (30-80 cents) per trip
VPN: $12 USD per month
Domestic flight: a one-way flight between major cities in China typically costs between ¥700-¥1200 ($100-$170). However, when seats are tight they can get over ¥2000 ($280)
Domestic high speed train: high speed trains pretty much connect all the major cities and many of the smaller cities. The cost varies, from less than ¥70-¥150 ($10-$20) for 1-3 hour trips, to ¥500-¥700 ($70-$100) for trips taking 5 or more hours
Hotel: ¥300-¥500 ($40-$70) per night in tier-1 cities (Beijing, Shanghai, Guangzhou, Shenzhen), and ¥100-¥300 ($15-$40) per night in non tier-1 cities (all other cities). This is for a 3-star hotel. Add 30%-40% to this range for 4-star hotels.
Airbnb: ¥200-¥600 ($30-$90) per night in tier-1 cities. ¥100-¥350 ($15-$50) per night for non tier-1 cities.
SIM card: depends on how much data you need, but ¥50-¥150 ($7-$21) will get you a few gigabytes of 4G data, which should be enough for most travelers
Tickets to attractions: public urban attractions like museums cost between ¥50-¥150 ($7-$20) per person. Public non-urban attractions like national parks (e.g. Huangshan) will cost between ¥200-¥350 ($30-$50) per person. Major private attractions will cost a lot more, usually over $50 USD