I did some math. By dividing 500 by 400, you get 1 1/4, so one robuck is worth 1.25¢.
Now, what are other currencies worth?
First, let's do Tix. This is easy, as 20 tix go into 1 robuck. This means that one tick is worth 0.0625¢ (1.25 / 20 = 0.0625).
Now, let's do some other currencies that aren't inherently Robloxian.
Pets GO states that 1,000 diamonds are equal to 1 robuck. This means that one diamond is 0.00125¢. If we assume a God Potion's RAP is exactly four million diamonds, then one of the potions would be worth exactly fifty dollars (0.00125¢ * 4,000,000 = 0.005¢ * 1,000,000 = 5¢ * 1,000 = 5000¢ = $50).
In Robot 64, you can buy tokens for 40 robux each. This means that one token is 50¢. As there are 39 token skins and 13 token hats (52 tokens), it would take a minimum of $26.