How would I make it cost 500 more Dominus's every rebirth?

Your best bet is to make a datastore script & have a intvalue that is called Rebirth. So Rebirth.Value=1 means you have no rebirths, which means it costs 500 domini to rebirth. Rebirth.Value=2 means you have 1 rebirth,which means it costs 1000 domini to rebirth. And so on. You can acheive this by using this in a local script:

local player = game.Players.LocalPlayer

local rebirthcost = 500 * player.leaderstats.Rebirth.Value

Then to take the money out use this in the local script:

player.leaderstats.Dominus.Value = player.leaderstats.Dominus.Value - rebirthcost

Also to add a rebirth.value use this line of code in the designated line:

player.leaderstats.Rebirth.Value = player.leaderstats.Rebirth.Value + 1

script.Parent.Yes.MouseButton1Click:connect(function()
	    if game.Players.LocalPlayer.leaderstats["Dominus's Collected"].Value < 5000 and db then
	        db = false
	        script.Parent.Question.Text = "YOU NEED ATLEAST 5,000 Dominus's TO REBIRTH!"
	        wait(3)
	        script.Parent.Visible = false
	        open = false
	        db = true
	    elseif db then
		----------------------------------------//RIGHT HERE\\--------------------------------------
	        game.ReplicatedStorage.Rebirth:FireServer()
	        script.Parent.Visible = false
	        open = false
	    end
	end)

P.S: In the datastore script make sure to set Rebirth.Value to 1 and not 0. You must have a leaderstat value called Dominus for this to work.

Hope this helped. If you need anymore help just ask.