As of right now, Dominus Rex is not available for purchase through the Roblox Avatar Shop. Since it was time-limited, you cannot acquire the item onto your account under normal circumstances. On the other hand, if you go on the internet, you might be able to find people who have purchased the original one and are now selling it. Nevertheless, it is also essential to keep in mind that they will not be selling it at the standard price at which it was sold. The prices now available on the market are much inflated compared to that level. I wouldn't advise you to buy the cosmetic item at such a high price unless you are a serious trader or significantly invest in Roblox. Other than that, I don't think it's worth it to spend such a high amount purchasing Dominus Rex in Roblox.