20 euros
Aallll the money in the world!!!!!!! 50 bagillion dollars!
There are alot of National Galleries but I take it that you mean London. If so, then there is no cost to enter, just a donation box as you walk round. There may be some incurred costs if you choose...
Refer to the link, below.
it costs $385 it includes fee for finger prints $80 and government fee which is $305.