This is really annoying problem and can be confusing for customers.

When you select a custom option on a product you can see +£3000 or -£3000 on the product display page.  works up to Magento 1.4 -1.9.2.4 not tested on 2.0 +

Before:

After:

The Solution

Navigate to JS/Varien/configurable.js

goto line 249 and comment out like below using // shown in red

formatPrice: function(price, showSign){
    var str = '';
    price = parseFloat(price);
    if(showSign){
        if(price<0){
            //str+= '-';
            price = -price;
        }
        else{
            //str+= '+';
        }
    }

    var roundedPrice = (Math.round(price*100)/100).toString();

    if (this.prices && this.prices[roundedPrice]) {
        //str+= this.prices[roundedPrice];
    }
    else {
        //str+= this.priceTemplate.evaluate({price:price.toFixed(2)});
    }
    return str;
},