I think it would be fine if the Robux price scaled to USD and also covered the 62% DevEx cut. However, the normal 30% cut wouldn’t be able to exist in this case, which could be potentially abusable, as discussed here:
Perhaps the Robux would also have to cover that as well, but price of Robux can change depending on where it was purchased, which means that the total offset it would have to cover is anywhere from 73.4% and 78.72%. You’d be paying nearly 3.76x more than if it were just USD, just to cover the offset, and that’s the best-case scenario, otherwise it would be about 4.7x more.
Here's the math for the above numbers:
Normally, you can either buy 500 R$ for $5 (rounded up) on PC, or 400 R$ for $5 (rounded up).
The percentages come from calculating the initial cost of said R$ and comparing it to the amount that the developer actually gets at the end of the process.
You start with either 400 or 500 R$ and multiply it by 0.7, this will take out the 30% tax, which would end up being 280 R$ and 350 R$ respectively. Then you’ll multiply both by the DevEx exchange rate, which can be calculated by $38 USD / 10k R$ = 0.0038. Given that, after multiplying the DevEx rate by the previous R$ amounts, you’ll get $1.064 USD and $1.33 USD respectively. Now divide both by the original $5 USD spent and multiply by 100 to get the percentage, and subtract 100 by that to get the percentage that Roblox takes. That’s 100 - (($Earnings USD / $5) * 5), which is 78.72% and 73.4% respectively.
Now, to get the multiplied times, you’ll need to divide 5 by the original $5 $5 / $Earned USD, which, given the previous numbers, would be about 4.7x and 3.76x (both rounded).