what price of an altcoin is, is not really that simple to say if it had X supply then it would be worth Y dollars. there are a lot of things that can affect that. from logical factors such a supply, the project itself, devs, adoption, ... to some illogical and weird things such as how much they hyped it up, how much they advertise it, which exchanges are listing it, is there easy wallets (light wallets and web wallets) available for it.

as for supply, they keyword that you should pay attention to is "circulating"! for example is 38.7B really circulating in the market or is it just a number of coins they have released but the real amount that is available in the market (to users) is a lot less?
i don't know much about ripple but i see it is not using the conventional methods of distribution such as mining reward, airdrops,... instead the company releases large amounts of the coin out of thin air whenever they want. for example they did it when XRP was at its highest this year.
or other coins have pre-mines where the devs own the majority of the coin but you see all of it as "circulating" whereas most of those coins never hit the market.