Reasonably priced rear bags advice needed??

[WindyChuffer]

If you mean the PVC holdall type Touratech ones, then Ortlieb make them and are ~40% cheaper.

Ortlieb also do other types of bags, all guaranteed submersible.

Traveldry also do Ortlieb style bags, are cheaper, but less durable being made of thinner material.

 

[original_concept]

Blimey... all this for a 400 mile stroll...

Get any compression sack, stick it inside a black bin liner, attach using cheapo bungee / string / wire / duct tape.

Go and enjoy.