You can use the demo as it should be installed there if wanting to see it from an Admin perspective. If from the end user, it is installed here and is the Resources tab on the menu.
As for cost, it is $60 (not including VAT if applicable) to purchase and then will add $15 to each annual license renewal that you do.
There is no "reduction" in cost depending on when you purchase it in your active license period as it's a flat cost. I would recommend you purchase it nearer the front of the 12 month license period instead of the end as all products are renewed at the same time.