The most expensive piece of equipment for water polo can be the pool unless you have access to open water (e.g. a lake or the sea).
The next most expensive piece of equipment are the goals; these will range from £1,000+ for a good set down to around £100 for a plastic set which are only good for practicing with.
Other equipment needed are a ball (around £40), a pair of trunks (£10 - £30) and a hat (£5 - £10)