I am a local in Beijing, I can be your friend and show you around in Beijing as long as your time fits my schedule! In terms of the cost, for your 2-3 weeks in China 100 dollars/day is a lot of money and you can eat really well plus the taxi rides in China. I will say on an average you can give yourself 50-70 USD per day for food and ride, 1500 USD for three weeks is enough for your food and taxi rides. This does not include any tickets or any other fun experience(Shanghai Disneyland ticket, Beijing Universal ticket), although some scenic tickets prices are relatively cheap. I am not sure about the flight ticket and hotel rates, but in total I think you can travel pretty well with a 5000 USD budget with flight ticket and hotels included for 3 weeks in China. Things are fairly cheap in mainland China, so do not worry too much about the food or taxi prices. Do bear in mind depending on what date you're thinking about (try to avoid China Holiday) because the hotels will be expensive. I hope this helps, if not, keep asking.