visit this page and click the bright blue, prominent button labeled "Convert to Robux":

https://www.roblox.com/my/account#!/billing

you will receive the exact amount you would get from the equivalent conversion rate of the closest "package"

Assuming this works like any other tiered package system, wouldn't this make you still lose more money due to the lower tiers being less efficient?

Not really. The lower tiers are only less efficient on platforms that require purchase through an app store or other payment processor that takes a large cut. Buying through a gift card or through the Roblox website or non-Microsoft Store PC app gives more Robux for the same price <https://www.roblox.com/upgrades/robux>. These increased rates are also balanced to prevent the lower tiers from being less efficient: the package for 800 goes to 1000, whereas 10 000 goes to 11 000.

There's an option buried somewhere in there to convert left-over real dollars into robux. They don't make it too prominent else no-one would be sitting there looking at the expensive robux options that offer (mostly, but NOT always!) a better exchange rate.

Scams and grifts from top to bottom.

Fortnite does that as well. Annoying dark pattern.

I mean, League of Legends does that as well. For example you can buy X RP, but a champion or skin or whatever costs either less or more, never the exact amount. I have seen this in hell of a lot of places as well.