Regardless of price the conversion for an 1/8th has already been stated.
1/8 = 3.5 grammes and refers to 1/8 of an ounce (an ounce being 28 grammes approx) there are no deviations from this conversion.
As far as prices are concerned they vary widely from place to place but if anyone ever asks me for £30 for an 1/8 I'd tell him to go fuck himself.