It varies from shop to shop, for instance a 12 litre fill at one shop near me is £1.50, at another a couple of miles further away is £3.
Visual test are usually around the £20-£25
Hydro's I'm guessing at £30-£35
Just bear in mind that if the tank fails you get charged for disposal but that said I have a 10+ year old tank that used for diving and it's been fine...
From what you've said your's might and I stress might be up for a hydro test, just take it to you local dive shop and ask them.
Bear in mind a tank with a full test is probably worth more than £20 over a tank that's 6 months out of test. Swings and roundabouts mate....