For the mayoral election, it's a £10k deposit, returnable if you get more than 5% of first preference votes. There's also a £10k fee (sorry, contribution to costs) if you want a one page election address to be included in the official booklet sent to all voters.
For the assembly, it's a £1k deposit for constituency candidates (5% threshold to get it back), and a 5k deposit per party, covering everyone on the list, for the London-wide seats (2.5% threshold).