Roblox Premium is a subscription-based model that is more cost-efficient than buying Robux outright. For example, if one buys Robux directly, then a $4.99 purchase will get you 400 Robux.

Alternatively, you can always convert your riches into Roblox Credits — to later convert into Robux or buy a Roblox Premium membership. Roblox is a time sink worth every penny you've spent.