There are two types of cover
1) Return to invoice – this pays the difference between the insurance payout and the original invoice price
2) Settlement Shortfall – this pays out only if the insurance payout is less than the finance settlement figure.
Most dealers will offer Return to Invoice – there really isn’t any difference in price between the two products but Return to Invoice is the product to have.
With regard the price, £500 is too much. Tell the dealer you like the idea and have a haggle. You could get it as cheap as £200-£250. The dealer needs to sell them and as long as they make profit you could get it cheaply.
Word of warning – if you do buy off the Internet you aren’t always getting the same level of cover, so check the small print. The products sold in dealers are normally provided by the big insurance companies but the Internet is littered with companies nobody has every heard of.