Which two airlines have substantially different products for a $5 difference?

If you didn't mean literally $5 perhaps you can give us some real examples of cost differences?

Personally I find so little difference between airlines in economy these days. And Premium Economy is nowhere close to a '$5' difference long haul these days

They don't, because competition erases those differences. It's a very competitive market. If people were willing to pay a little extra for a few more inches of seat pitch in economy, you'd have an airline that offered a few more inches of seat pitch in economy with slightly higher fares.