List<String> list = new ArrayList<>();
list.add(String.valueOf(currStack.getDisplayName()));
list.add("$" + (String.valueOf(tile.getItemCost(slot))));
FontRenderer font = stack.getItem().getFontRenderer(stack);
net.minecraftforge.fml.client.config.GuiUtils.preItemToolTip(stack);
this.drawHoveringText(list, x, y, (font == null ? fontRendererObj : font));
net.minecraftforge.fml.client.config.GuiUtils.postItemToolTip();
Try adding the cost after the tooltip event, as NEI might tinker with that.
Also, not that you should, but you should probably use JEI instead of NEI, because NEI is doing some unnecessary core-modding which JEI doesn't. And as we all know, core-modding is bad.