I picked up a few old Japanese buxus from the side of the road and cut them way back. Obviously they need a fair bit of work but the trunk is already there.

What would you value them for with a little bit of work?

At the moment it is impossible to tell - you will need to SHOW the work that you have done before they can be valued.
What I mean by this is that if you completely wreck the trees then they will be worth less than if you do good work on them.
Until work is done, you will have to wait for evaluation

It is not impossible to tell what the current value is - it's that what people are willing to pay for it, based on the current status and potential and species.

That means the value will be based on the trunks only. Branches are underdeveloped or absent, the wiring ads nothing to the value.

The first one seems to have nice tapering, the second one seems quite fat, but doesn't seem tapered.

Based on what I see on the photo I would probably want to pay 10-15 euro for the first one, 7,50 for the second one, max.