It depends on the maker and the weapon, but you’re looking at roughly between £45 for a Eldritch Armouries double-edged sword, to ~£65 for a similar Saxon Violence sword (both vendors have stalls at Empire).
If you’re just planning to use a single sword, there’s a very good chance someone will be able to lend you one - it’d be worth asking on the Brass Coast facebook page.
Also worth noting: If you are planning on going on one of the large battles as your character, you also have to “monster” the other - ie; play as one of the enemies (usually Orcs) that the Imperials will be facing. This is organised on a nation-by-nation basis, so you’ll be with people you may have already met in the Brass coast. You don’t need any special costume for this, but you will need a weapon, and ideally some dark/neutral clothes, like a simple pair of cotton trousers + shirt (you’ll be loaned an Orc mask and a plastic breastplate for the battle).