import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
public class Price {
	   private static final String BASE = "https://api.rsbuddy.com/grandExchange?a=guidePrice&i=";
	    public int getOverallPrice(int itemID) throws IOException {
	        return this.parse(itemID, "overall");
	    }
	    public int getBuyingPrice(int itemID) throws IOException {
	        return this.parse(itemID, "buying");
	    }
	    public int getSellingPrice(int itemID) throws IOException {
	        return this.parse(itemID, "selling");
	    }
	    private int parse(int itemID, String choice) throws IOException {
	        String line;
	        URL url = new URL("https://api.rsbuddy.com/grandExchange?a=guidePrice&i=" + itemID);
	        BufferedReader file = new BufferedReader(new InputStreamReader(url.openStream()));
	        String price = null;
	        while ((line = file.readLine()) != null) {
	            if (!line.contains("{")) continue;
	            price = line.trim();
	        }
	        price = choice.equals("buying") ? price.substring(price.indexOf(",") + 10, this.nthOccurrence(price, ',', 1)).trim() : (choice.equals("selling") ? price.substring(this.nthOccurrence(price, ',', 2) + 11, price.indexOf("sellingQuantity") - 2).trim() : price.substring(price.indexOf(":") + 1, price.indexOf(",")).trim());
	        file.close();
	        return Integer.parseInt(price);
	    }
	    private int nthOccurrence(String str, char c, int n) {
	        int pos = str.indexOf(c, 0);
	        while (n-- > 0 && pos != -1) {
	            pos = str.indexOf(c, pos + 1);
	        }
	        return pos;
	    }
}
Then you use it like so: 
public class Main extends Script{
    Price price = Price();
    int priceOfItem;
    public void onStart(){
    priceOfItem = price.getSellingPrice(ITEMID);
    }
}