Does anyone know a rough cost for secondary glazing per m²?

Question:

Answers:
This is not a direct answer to your question but it may be worthy of consideration and provide an approach you had not considered.
1] You would need to be a bit handy or know someone who is.
2] You do not indicate the type of window you have. If you have a casement or pivot window with a large rebate and large existing glazing bead then you might be able to reglaze using a sealed double glazed unit with a minimal gap say 9mm or 10mm which you install in to the existing frame,thus dispensing with the concept of secondary glazing.
3] It is necessary to use BUTYL mastic not putty and you would probably need to have new glazing beads run by your timber merchant.
4] I have successfully undertaken this exercise on existing Pivot windows where the timber was in GOOD CONDITION. (All be it that the 10mm gap betrween panes may not bring your window up to current Part L of the Building Regulations requirements for window U Values. )The improvement in comfort was significant and the demise of condensation on the glass appreciated.
5] It could only work on traditional sliding sash windows if the additional weight of the glass in the sashes could be accommodated by the provision of additional counterweights in the counterweight chambers. It is likely that the weight chambers at the side of the windows are not big enough to get more weights in to, thus rendering this solution unfeasible for sliding sash windows.
6] The cost of 4mm double glazed unit is £36/M2 .This is how long it took me (not an experienced hand by any means).
Approximately 2 hours labour to remove the old moving (hinged or pivoting)sash from the main frame ,4 hours to remove glass and Putty and 4 hours re-install the sealed double glazed unit.
If this approach seems appropriate, I suggest you do one pane as a test ; it may be necessary to use spacers to avoid the silver glazing tape"sinking" behind the rebate. Do a full size drawing: an elevation and a section through the sash to assist with design of the new glazing beads.
I reglazed 15 sashes (2 panes per sash) for about £600 excluding my own labour.
Probably saved myself about £2500
Good luck