It is usually fractured solder on the RCM - Dj in Arizona can fix that if you send him your RCM, for around $40. When the RCM fails, the pump wont run and you get a P0401.
Of course, your 'mechanic' probably never even tried to see if closing the relay makes the pump run - he just assumed the pump was bad - and in ten years here on the forum, I can tell you that I remember maybe TWO times is was the pump, every other time, it was the RCM. Again, easy fix.

In any event,
That valve is not $1500, (well, thru Chrysler it probably is). In that case, tell your 'mechanic' to order the Bosch part for the SLK320.
If the valve-pump IS bad, even with labor, it should not go over maybe $500.

But he WAS right, if you are not doing emissions, who cares? (Well, actually, you should fix it, cause you have a CEL, if another code is generated for some other issue, you will never know, cause the light is going to be on most of the time due to the P410.)