This is how you can approach it :
Daily Budget = (Cost per acquisition from other channels)* (expected number of conversions per day)
For example your CPA (organically) is $5. If you are starting new on Facebook Ads, then you can assume the same CPA for facebook.
And you want to do 4 sales per day through facebook.
In that case you can have your daily budget as $20($5*4). After you have made enough sales through facebook, you will have a clear idea regarding it's CPA and accordingly you can vary your budget in the next iteration.
Hope this helps
Orangetwig :- Drive traffic to your store by social media