Aye! There's never any shortage of people who bought a cheaper one but usually it was 15 years ago..
The reality is that a Bosch Silver or the latest Calcium type batteries do cost £100. It's a big old beast in the GTV; not many batteries come bigger than that (go and check out the ranks of batteries at Halfords and you'll see what I mean).
If you take it for a battery test, make sure you go somewhere you trust or they'll touch you for a new battery and you'll be back on here posting next week.
You can do it yourself if you know someone with a multimetre.
Measure the voltage across the terminals. If it's less than 12.6V you need to charge it up. Charging should take the reading to 14V and then it will settle to 12.6V. If the voltage does not stay at 12.6V (falls to 11V) then it's knacked.
To check that it's not being drained by some electric component, disconnect the battery while you do the test.
To check if the battery is being charged up okay, reconnect it and then start the engine up. With the revs around 1500rpm, the voltage across the battery terminals should be 13.5V or thereabouts. If it's 12.6V then your battery is not being charged up/enough, which is why it's going flat.
To test that the battery is not being drained, you need to switch everything off and measure the Ampage across the terminals. I dunno what readings you should get... but all that's running is the clock and the immobilisor, so that should be virtually nothing. Your alarm may draw more (especially if it's overdoing it and causing the drain) but I dunno if you can set it on with the boot open.. someone on here will probably have done it and know for sure.
If you haven't got a multimeter, buy one anyway. They never wear out and you will need one a few times in your life.