Looking at a routing between DUB and LAX via LHR in Prem Y and ultimately wanting to use Avios to get into CW

When I get the option to upgrade into CW, it comes up with 34,000 Avios, but the taxes etc are about $300.00 more than if simply buying Prem Y

Any reason behind this. I thought it was a simple question of buying Prem Y and upgrading using Avios

Tks