It's all subjective. Notionally, the closer you are to a desired level of elite status, the more "valuable" the TPs.

In practice, you aren't going to find too many opportunities to acquire TPs for less than £2-3. So paying £1.6 per TP on a trip you are already taking - with the added benefit of flying J instead of Y - and it's a pretty straightforward decision for anybody who thinks they might come up short on renewal or upgrading their level of status.