Plan shape
A monthly pack is cheaper per GB only if you use it before it dies. This is that sentence as arithmetic.
How to use it
Pack wins when used GB × PAYG rate > pack price, and the unused GB would have expired.
Use the plan you would actually buy, not the enterprise floor.
Decodo’s public PAYG is $4/GB plus VAT. Type the rate on the page you are reading.
If you will not use the pack, the lower per-GB line is fiction.
A non-expiring pack can keep leftover GB. A monthly plan cannot. The tool asks which one you have.
Add them before you type the rate. A $4 sticker plus VAT is not $4. A $4 coupon on an $8 list is not the list. The arithmetic is only as honest as the inputs.
Published rates are a snapshot. Estimates are arithmetic, not invoices. Confirm the live price, VAT and expiry with the provider before you pay.
FAQ
Per GB, often, if you use the allowance. If you use half and the rest expires, pay-as-you-go was the cheaper product.