How big does the transaction fee need to be if the transaction is approximately 50mbtc ($5000 usd)?
I really don't understand this bitcoin mempool stuff
You should probably read up on the basics: total fees are based on the transaction size in (v)bytes, not on the amount in Bitcoin. If you enable the Coins tab in Electrum, you can manually select which inputs to use. The more you select, the more you'll have to pay in transaction fees.
See
this topic on how to deal with small inputs.