This will probably be off a couple cents because of the Roblox teams' lack of consistency. (For example, 80 robux can be bought through mobile devices for 0.99$ Yet if you buy it through Rixten it is 1$, This causes me to have to round the difference between robux and dollars to multiply the robux by that to get the amount of money it is worth.
thx Ol_Entertainment, you a g I'm not sure if this is correct tho lol