I'm very seriously looking at building a Dax 427, and have been pricing bits up.

I was amazed at the price of Sierra steering columns, anything from £20 to £200.

So thought I'd better strike whilst there are some on offer, is 50-80 a reasonable price for a Mk1 with shrouds switches and keys?

Yes that seems reasonable .

Shrouds on Their own seem to fetch £25 upwards

I have a Sierra column with 1 key, lock, stalks and support bearing that I removed from my Haynes and replaced with an MX5 one, no shrouds, pm me with an email and I'll send pics, £55 posted