What do you have to pay to respec?

There is now an option to respec your skill tree, but the price to pay for it isn't clear. Is it some kind of calculation where you get back your skill points and a certain percentage of the cash spent ?

Apparently, you get back all of your skill points and 50% of the cash spent. From this guide:

If you want to respec your skill points, you can either respec a certain tree (not branch) or just all four trees if you would like. The number of cash shown at the respec button is not what you have to pay but what you will get back for respecing. The amount is 50 % of the cash you used to spend on the skill tree(s).