What does it cost to send XRP?
Sending XRP costs the XRP Ledger network fee, a hundredth of a cent, and nothing else. XRP Mobile does not add a fee on payments.
An XRP payment pays the ledger's network fee, currently about 0.00001 XRP, a hundredth of a cent. There is no XRP Mobile fee on top. The amount screen shows what the recipient gets and the fee before you confirm.
For comparison, swaps and cross-chain sends have their own costs, because they involve other networks. See What does a swap cost?.
What does a swap cost?
A swap pays the XRP Ledger network fee, a provider fee that is shown on the confirmation screen and already included in the quote, and accepts up to 1% slippage. There is no separate XRP Mobile fee.
What is a destination tag?
A number that exchanges and services attach to your deposit so they know which customer the XRP belongs to. It is only needed when the recipient asks for it, and getting it wrong means the exchange has to recover the deposit.