How much does an R4 card for DS cost?

The cheapest i have seen was for £5.42/$9.50 (not including P&P or MicroSD Card)

There are too many r4 ds card in the market currently. The cheapest r4 ds card cost about 5EUR. But it only support DS and DS lite. The famous card is r4i sdhc 3ds. It support Nintendo 3DS,DSi,DS. So, I suggest you to buy a r4i sdhc 3ds card.