I've purchased 4 recently,
A very nice clean slant grip for £100
A superb Straight grip for the same
A slightly tatty straight grip for £80
and a tatty but shooting pre war slant for £40 (auction)
I've also seen them on sale for a lot more, There are a lot of them about so there are normally quite keenly priced examples readily available with the higher priced examples remaining un-sold, personally I favour the MK1 over all other models but the Mk1 Premier or the Senior appear to get the better prices,