No 7 MK I magazine for sale, how much should I pay

Gents

I am trying to put together a No 7 MK I rifle from one of the old Springfield sporters barreled incomplete actions. One of the items I lack is the correct magazine. I have a substitute magazine, but have been offered a correct magazine housing and a correct BSA magazine insert by a fellow that had one (and a lot of other neat parts I am lusting after such as SMLE MK I wood and early No 4 parts). My question is price, he asked me to pay him what it was worth. I was guessing that a fair price is 150 dollars for the entire assembly. Is that fair, I am hurting myself or am I ripping the fellow off? I would like to offer 100 bucks but that seems a tad low

As a price point the missing parts other than the magazine are going to cost around 350 (when and if they become available). The rifle cost me 400 and the substitute BSA magazine cost me 45 dollars, so I do not really need it but would like to have the actual correct one if possible. Also my substitute magazine has to be modified and having the real one will allow me to ensure I modify the substitute magazine correctly.

Thanks in advance.

A Brit No7 mag housing and 5 shot insert is a bargain at $150. I would expect to pay $250 and up as these are rarely offerred to the market. don't see you going wrong on your investment $'s in the No7.