I know a lot about traffic signs, I used to be in street lighting/traffic sign business.
Is it a 600mm circle or triangle?? It does make a difference to what I would have normally paid.
In any case, I would expect to pay in the region of £27.00 to £35.00 maximum.
Be careful what you say that you are supplying them with though as the specs for traffic signs are now very high.
You said you supplied channell on the back, how did you fix it??? Traffic signs these days (If on the highway) shouldnt have its face pierced i.e.old fashioned rivets dont do anymore.
If you interested Mail/PM me and I will give you a more accurate guide on standard sign prices (Triangles and circles 300mm up to 900mm).
It will be from memory mind you as Ive been out of the business for a month or three.