Under the TSO granted to the new 406 ELT manufacturers, a greatly beefed-up mounting plate is required, because the new ELT's mounting must withstand 100G's.
Do you have a reference, 411A? I don't know any better myself but this kind of thing is going to be highly airframe dependent.
I've had silly-price quotes for installing a 406MHz unit too (N-reg), and amazingly silly prices quoted for the DER stuff (which is usually done by
DER Associates for about $300), but it would not suprise me if your experience comes from a certain UK avionics shop whose way of working is well known in this business.
For example, the Artex ME-406 replaces the old ELT200 using (AIUI) the same mounting points, and no new bracket is needed.
FWIW, I have never heard of a 100G requirement, and if an ELT weighs say 1kg that is only a 100kg (1000N) of force which is nothing for a piece of 1.5mm aluminium.