The default fee for a transaction is 0.00001 XRP.
But the fee is not fixed. Quoting the wiki:
Transaction fees are adjusted based on two factors: base fee and load
base fee: long term fee adjustments
Since transaction fees are
paid in XRP and the price of XRP may change overtime, to keep the
price of transactions affordable, Ripple uses an adjustable "base fee"
to specify the cost of transactions.
load fee: short term fee adjustments
The "load fee" can rise quickly to protect the network
from attacks and keep the network functional under heavy load.
The fee schedule is changed using consensus, quoting. D.Schwartz:
A node can introduce a transaction to change the base fee. If a
trust-weighted majority of nodes vote "yes" on the transaction, it
will be incorporated into the ledger and the fee will be changed.