How expensive is it to convert to a Salt Water pool?

That depends on the size of the pool And what sort of system you intend to install.the conversion only involves fitting the element to the filtration system and the electronic controls then putting the required amount of salt in the pool.

You have a choice in the type of salt you use

1 sodium chloride or common salt you will find this the easiest to get hold of or

2 Magnesium and potassium chloride available from magnablu which are much better for you and good for your garden.

I suggest that you speak to a local pool professional about your options.