Your daily budget is entirely dependent on what you feel comfortable spending. The more you spend quickly, the faster you acquire sales data. Lower campaign budgets means it will take longer to acquire that sales data. The quicker you can acquire sales data, the quicker you can find the converting and profitable search terms and use that information to run successful campaigns. Here is more information regarding daily budget:
Reminder: Daily budget is not a true daily budget. It’s a monthly average. If you want to spend ~$300 a month, you’d set your budget at $10 a day. However, Amazon could spend $12 on one day and then $8 the day after because you had leftover budget from the day before.
Account level: You can control exactly what your cap is per day, or you can let your campaign budgets dictate spend. Example: 3 campaigns running on a $20 per day budget, you set an account cap at $50 per day then all campaigns will stop when the account total reaches $50.