Definitely not that simple.

When the XM came out in 1989, the list price ranged from ~£20k to ~£30k.
When the XM was discontinued, the list price ranged from ~£20k to ~£30k.
Upper-spec C5s today have list prices ranging from ~£20k to ~£30k.

As a gut-feel, I'd say "Somewhere a bit below a pikey-spec C1" - same as it used to sit a bit below a pikey-spec Wheezer. List for a a pikey-spec C1 starts at £8845 - so if we say ~£7k for a Spesh to ~£8k for a Charlie, that "feels" about right. And, as pointed out, the iffy tiddlers still squeek in beneath.