I don't know where you got the idea that a 1 gig SD card cost £200, as Belatucadrus says if its that price there must be something on it as cards are not the expensive. In fact when the 1 gig cards first appeared they weren't that expensive and they are cheaper now.
You say your satnav has an SD slot, I suggest you check out the manual for it and the website (I presume they have one) because its possible it won't take a card as large as that. Some devices have a limit and can only read a certain size of card. Better to be sure before purchase.
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