Firstly the price depends on the power of the battery and the more power the more it costs. A test by the CAMH some time back suggested that many sold as leisure batteries were nothing more than car batteries with a different label.
Frankly you will normally get what you pay for and the cheap on line ones are likely to be rubbish. Do buy a sensible priced battery from a reputable source to try and avoid getting stung. As one firm says buy cheap, buy twice.
If you do use the battery only to even out the power supply though and do not have a mover or go off grid the lowest power model will do the job. If you go off grid or use a mover though, you need something better.