How much should I bid?

You should bid the amount that you are able to spend and still drive a profit. So, basically, it’s the margin between cost and profit. You take that amount and you use that for your cost cap bidding in the Ads Manager. 

My delivery is gone after I changed my bid.

Make sure your bid is high enough to get delivery going. Try not to be too cheap here. Remember that Facebook’s auction always looks for the cheapest conversions. For example, if your bid is $20, the auction will start looking for all conversions below this amount. When there are no conversions left, it will look for conversions at a higher price until it has exhausted all opportunities that match your bid, budget and targeting. 

Bidding strategy best practices

Don’t know how much you should be bidding? → Autobid

Know what you want to pay for a conversion? → Lowest cost with cost cap

With cost cap, you control the cost.

More on bidding strategies here.