The short explanation
Cash-out value is not purchase value.
If you paid for 1,000 Robux, Roblox says those purchased Robux do not count as eligible Earned Robux. The official DevEx rates describe what qualifying creator earnings may be worth after Roblox review.
For a purchase answer, use the amount shown at the official checkout for your account and location. US sales tax may be added to the displayed price.