The cost is down to the type of battery fitted, a lot of higher end cars are fitted with AGM batteries these days, a G14 for a audi for example is £180, but they are seriously better than std type batteries in ah and especially CCA and do not lose power anything like as fast as a std battery when not used, leave it 2 months and it will start up again.
They can of course be specced with std lead/acid batteries.
tabetha