Currently I have a mortgage on my house, the mortgage is £2000.

I am moving to another city, but dont wish to sell my house, the rental value is £1500.

Does it make sense for me to 'cover' the £500?

The house is worth £600,000 and I have 20 years left on my mortgage.

In my mind, I'll have an asset worth £600,000 by only paying £500 a month (£120,000 = £500 * 12 months * 20 years). 

But I'm not sure if that's a good way to look at it.