I don't know about "a great deal of money" but the price of an average condition Type 38 rifle would be around $175-250, with 20-25% off if the "mum" is ground. Top value for one in like new condition with the "mum" would be around $600, again with 20-25% off for the ground receiver.
A few Japanese rifles, like sniper rifles, paratrooper rifles, or the Type 44 carbine are in enough demand that removal of the "mum" will matter less.
(The above prices are retail, in other words what you would expect to pay when buying from a dealer. What a dealer will pay you is a different story.)